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2019
Dayton, PK, Jarrell SC, Kim S, Parnell PE, Thrush SF, Hammerstrom K, Leichter JJ.  2019.  Benthic responses to an Antarctic regime shift: food particle size and recruitment biology. Ecological Applications. 29   10.1002/eap.1823   AbstractWebsite

Polar ecosystems are bellwether indicators of climate change and offer insights into ecological resilience. In this study, we describe contrasting responses to an apparent regime shift of two very different benthic communities in McMurdo Sound, Antarctica. We compared species-specific patterns of benthic invertebrate abundance and size between the west (low productivity) and east (higher productivity) sides of McMurdo Sound across multiple decades (1960s-2010) to depths of 60 m. We present possible factors associated with the observed changes. A massive and unprecedented shift in sponge recruitment and growth on artificial substrata observed between the 1980s and 2010 contrasts with lack of dramatic sponge settlement and growth on natural substrata, emphasizing poorly understood sponge recruitment biology. We present observations of changes in populations of sponges, bryozoans, bivalves, and deposit-feeding invertebrates in the natural communities on both sides of the sound. Scientific data for Antarctic benthic ecosystems are scant, but we gather multiple lines of evidence to examine possible processes in regional-scale oceanography during the eight years in which the sea ice did not clear out of the southern portion of McMurdo Sound. We suggest that large icebergs blocked currents and advected plankton, allowed thicker multi-year ice, and reduced light to the benthos. This, in addition to a possible increase in iron released from rapidly melting glaciers, fundamentally shifted the quantity and quality of primary production in McMurdo Sound. A hypothesized shift from large to small food particles is consistent with increased recruitment and growth of sponges on artificial substrata, filter-feeding polychaetes, and some bryozoans, as well as reduced populations of bivalves and crinoids that favor large particles, and echinoderms Sterechinus neumayeri and Odontaster validus that predominantly feed on benthic diatoms and large phytoplankton mats that drape the seafloor after spring blooms. This response of different guilds of filter feeders to a hypothesized shift from large to small phytoplankton points to the enormous need for and potential value of holistic monitoring programs, particularly in pristine ecosystems, that could yield both fundamental ecological insights and knowledge that can be applied to critical conservation concerns as climate change continues.

2018
Wing, SR, Leichter JJ, Wing LC, Stokes D, Genovese SJ, McMullin RM, Shatova OA.  2018.  Contribution of sea ice microbial production to Antarctic benthic communities is driven by sea ice dynamics and composition of functional guilds. Global Change Biology. 24:3642-3653.   10.1111/gcb.14291   AbstractWebsite

Organic matter produced by the sea ice microbial community (SIMCo) is an important link between sea ice dynamics and secondary production in near-shore food webs of Antarctica. Sea ice conditions in McMurdo Sound were quantified from time series of MODIS satellite images for Sept. 1 through Feb. 28 of 2007-2015. A predictable sea ice persistence gradient along the length of the Sound and evidence for a distinct change in sea ice dynamics in 2011 were observed. We used stable isotope analysis (delta C-13 and delta N-15) of SIMCo, suspended particulate organic matter (SPOM) and shallow water (10-20m) macroinvertebrates to reveal patterns in trophic structure of, and incorporation of organic matter from SIMCo into, benthic communities at eight sites distributed along the sea ice persistence gradient. Mass-balance analysis revealed distinct trophic architecture among communities and large fluxes of SIMCo into the near-shore food web, with the estimates ranging from 2 to 84% of organic matter derived from SIMCo for individual species. Analysis of patterns in density, and biomass of macroinvertebrate communities among sites allowed us to model net incorporation of organic matter from SIMCo, in terms of biomass per unit area (g/m(2)), into benthic communities. Here, organic matter derived from SIMCo supported 39 to 71 per cent of total biomass. Furthermore, for six species, we observed declines in contribution of SIMCo between years with persistent sea ice (2008-2009) and years with extensive sea ice breakout (2012-2015). Our data demonstrate the vital role of SIMCo in ecosystem function in Antarctica and strong linkages between sea ice dynamics and near-shore secondary productivity. These results have important implications for our understanding of how benthic communities will respond to changes in sea ice dynamics associated with climate change and highlight the important role of shallow water macroinvertebrate communities as sentinels of change for the Antarctic marine ecosystem.

Edmunds, PJ, McIlroy SE, Adjeroud M, Ang P, Bergman JL, Carpenter RC, Coffroth MA, Fujimura AG, Hench JL, Holbrook SJ, Leichter JJ, Muko S, Nakajima Y, Nakamura M, Paris CB, Schmitt RJ, Sutthacheep M, Toonen RJ, Sakai K, Suzuki G, Washburn L, Wyatt ASJ, Mitarai S.  2018.  Critical information gaps impeding understanding of the role of larval connectivity among coral reef islands in an era of global change. Frontiers in Marine Science. 5   10.3389/fmars.2018.00290   AbstractWebsite

Populations of marine organisms on coral reef islands (CRI) are connected in space and time by seawater that transports propagules of plants, animals, and algae. Yet, despite this reality, it is often assumed that routine replenishment of populations of marine organisms on CRI is supported by locally-sourced propagules (hereafter, larvae). Following large disturbances, however, distantly-sourced larvae from less disturbed CRI within a regional meta-population are likely to be important for local population recovery, but evaluating the roles of locally-vs. distantly-sourced larvae remains difficult. While larval sources are relatively well-known for some fishes, they remain virtually unknown for most taxa, particularly those associated with the benthos, as exemplified by scleractinian corals. This review focuses on reef recovery and larval connectivity. Using corals as examples, we argue that CRI can serve as natural laboratories in which studies of these issues can enhance understanding of coral reef community dynamics under future disturbance regimes. Rather than focusing on synthesizing empirical data, we focus on the capacity for CRI to realize their potential in this research area, concluding that progress is impeded by the limited breadth, detail, and spatio-temporal concordance

2016
Knee, KL, Crook ED, Hench JL, Leichter JJ, Paytan A.  2016.  Assessment of submarine groundwater discharge (SGD) as a source of dissolved radium and nutrients to Moorea (French Polynesia) coastal waters. Estuaries and Coasts. 39:1651-1668.   10.1007/s12237-016-0108-y   AbstractWebsite

Previous work has documented large fluxes of freshwater and nutrients from submarine groundwater discharge (SGD) into the coastal waters of a few volcanic oceanic islands. However, on the majority of such islands, including Moorea (French Polynesia), SGD has not been studied. In this study, we used radium (Ra) isotopes and salinity to investigate SGD and associated nutrient inputs at five coastal sites and Paopao Bay on the north shore of Moorea. Ra activities were highest in coastal groundwater, intermediate in coastal ocean surface water, and lowest in offshore surface water, indicating that high-Ra groundwater was discharging into the coastal ocean. On average, groundwater nitrate and nitrite (N + N), phosphate, ammonium, and silica concentrations were 12, 21, 29, and 33 times greater, respectively, than those in coastal ocean surface water, suggesting that groundwater discharge could be an important source of nutrients to the coastal ocean. Ra and salinity mass balances indicated that most or all SGD at these sites was saline and likely originated from a deeper, unsampled layer of Ra-enriched recirculated seawater. This high-salinity SGD may be less affected by terrestrial nutrient sources, such as fertilizer, sewage, and animal waste, compared to meteoric groundwater; however, nutrient-salinity trends indicate it may still have much higher concentrations of nitrate and phosphate than coastal receiving waters. Coastal ocean nutrient concentrations were virtually identical to those measured offshore, suggesting that nutrient subsidies from SGD are efficiently utilized.

Edmunds, PJ, Leichter JJ.  2016.  Spatial scale-dependent vertical zonation of coral reef community structure in French Polynesia. Ecosphere. 7   10.1002/ecs2.1342   AbstractWebsite

We tested for depth-dependent vertical zonation in coral reef community structure with surveys conducted in 2013 at 10 m and 27 m depth off the islands of Moorea, Tahiti, Maiao, and Tetiaroa, which are distributed over similar to 3500 km(2) of the tropical south Pacific. Benthic communities were censused using photoquadrats to obtain percentage cover, first by functional groups (scleractinians, macroalgae, and crustose coralline algae, algal turf, and bare space combined [CTB]), and second by genus for scleractinians and Millepora. Virtually every aspect of community structure differed between the two depths, but the effects were dependent on spatial scale of investigation. Interactive effects of islands and depths determined the abundance of functional groups, as well as seven of eight common coral genera. On the two islands at which two sites were censused, the abundance of functional groups also differed between sites, and was affected by depth x site interactions. The spatially variable effects of depth on community structure underscores the importance of context-specific synergy between biotic and abiotic factors in driving these patterns. Spatial scale-dependent vertical zonation also shows the potential for these effects to be modulated by the changes that have affected coral community structure on contemporary reefs over the last few decades.

MacTavish, AL, Ladah LB, Lavin MF, Filonov A, Tapia FJ, Leichter J.  2016.  High frequency (hourly) variation in vertical distribution and abundance of meroplanktonic larvae in nearshore waters during strong internal tidal forcing. Continental Shelf Research. 117:92-99.   10.1016/j.csr.2016.02.004   AbstractWebsite

We related the vertical distribution and abundance of nearshore meroplankton at hourly time scales with internal tidal wave events. We proposed that significant changes in plankter abundance would occur across internal tidal fronts, and that surface and bottom strata would respond in opposite fashions. First mode internal tidal bores propagating in the alongshore direction were detected in water-column currents and baroclinic temperature changes. Surface and bottom currents always flowed in opposite directions, and abrupt flow reversals coincided with large temperature changes during arrival of bores. Crab zoeae and barnacle cyprids were more abundant in the bottom strata, whereas barnacle nauplii showed the opposite pattern. Significant changes in vertical distribution and abundance of target meroplankters occurred across internal tidal fronts, especially for crabs at depth, with surface and bottom organisms responding in opposite fashions. Changes in plankter abundance were significantly correlated with current flows in the strata where they were most abundant. The manner in which plankters were affected (increasing or decreasing abundance) appeared to be modulated by their vertical position within the water column. The significant differences found at the high frequencies of this study, maintained across sampling days, suggest that nearshore meroplankton populations may have greater and more consistent temporal and vertical variability than previously considered. (C) 2016 Elsevier Ltd. All rights reserved.

Hanson, KM, Schnarr EL, Leichter JJ.  2016.  Non-random feeding enhances the contribution of oceanic zooplankton to the diet of the planktivorous coral reef fish Dascyllus flavicaudus. Marine Biology. 163   10.1007/s00227-016-2849-3   AbstractWebsite

Combining gut content analysis and sampling of ambient zooplankton, we examine departures from random feeding in a planktivorous coral reef fish and explore the effects of apparent non-random feeding behavior on the relative contribution of oceanic versus reef-associated zooplankton to fish diet. The planktivorous damselfish Dascyllus flavicaudus appears to exhibit strong positive electivity for oceanic copepods including Candacia spp. and copepods from the families Oncaeidae and Corycaeidae and consistent negative electivity for cyclopoid copepods (Oithonidae). In total, prey taxa categorized as oceanic in origin contributed 10-76 % of total zooplankton biomass in fish guts. The summed contribution of oceanic prey taxa to fish diet was significantly higher than expected under a model of random feeding based on the availability of oceanic versus reef-associated prey as sampled by zooplankton net tows. The feeding behavior of D. flavicaudus appears to be visibility-selective rather than or in addition to size-selective, as electivity across prey taxa could not be explained by differences in prey size alone.

Edmunds, PJ, Leichter JJ, Johnston EC, Tong EJ, Toonen RJ.  2016.  Ecological and genetic variation in reef-building corals on four Society Islands. Limnology and Oceanography. 61:543-557.   10.1002/lno.10231   AbstractWebsite

We quantified benthic community structure on shallow (10 m isobath) reefs separated by 3-130 km on four islands in the south Pacific, and evaluated the roles of disturbances vs. coral recruitment as causes of spatial heterogeneity. Reefs were surveyed in 2013 on Moorea, Tahiti, Tetiaroa, and Maiao, with community structure sampled at two sites on each island using photoquadrats. The effects of coral recruitment on population structure were evaluated through genetic analyses of Pocillopora on three of the islands. Benthic community structure with functional group resolution differed among islands and generally among sites, but coral community structure (generic resolution) differed among islands, but generally not among sites. Genetic analyses of Pocillopora using the open reading frame of host mtDNA revealed varying relative abundances of Pocillopora meandrina/Pocillopora eydouxi, Pocillopora verrucosa, Pocillopora effuses, and two unnamed haplotypes on each island. These results suggest that corals on each island represent unique samplings of genetically discrete larval assemblages rather than random samplings of a single larval assemblage. Together, our findings emphasize the extent to which coral community structure varies over a scale of < 200 km, and suggests that recruitment from spatially discrete pools of coral larvae plays an important role in creating spatial variation in community structure, even where reefs are connected by prevailing currents.

Switzer, RD, Parnell EP, Leichter JL, Driscoll NW.  2016.  The effects of tectonic deformation and sediment allocation on shelf habitats and megabenthic distribution and diversity in southern California. Estuarine, Coastal and Shelf Science. 169:25-37.   10.1016/j.ecss.2015.11.020   Abstract

Landscape and seascape structures are typically complex and manifest as patch mosaics within characteristic biomes, bordering one another in gradual or abrupt ecotones. The underlying patch structure in coastal shelf ecosystems is driven by the interaction of tectonic, sedimentary, and sea level dynamic processes. Animals and plants occupy and interact within these mosaics. Terrestrial landscape ecological studies have shown that patch structure is important for ecological processes such as foraging, connectivity, predation, and species dynamics. The importance of patch structure for marine systems is less clear because far fewer pattern-process studies have been conducted in these systems. For many coastal shelf systems, there is a paucity of information on how species occupy shelf seascapes, particularly for seascapes imbued with complex patch structure and ecotones that are common globally due to tectonic activity. Here, we present the results of a study conducted along a myriameter-scale gradient of bottom and sub-bottom geological forcing altered by tectonic deformation, sea level transgression and sediment allocation. The resulting seascape is dominated by unconsolidated sediments throughout, but also exhibits increasing density and size of outcropping patches along a habitat patch gradient forced by the erosion of a sea level transgressive surface that has been deformed and tilted by tectonic forcing. A combination of sub-bottom profiling, multibeam bathymetry, and ROV surveys of the habitats and the demersal megafauna occupying the habitats indicate (1) significant beta diversity along this gradient, (2) biological diversity does not scale with habitat diversity, and (3) species occupy the patches disproportionately (non-linearly) with regard to the proportional availability of their preferred habitats. These results indicate that shelf habitat patch structure modulates species specific processes and interactions with other species. Further studies are needed to examine experimentally the mechanics of how patch structure modulates ecological processes in shelf systems. Our results also provide further support for including multiple spatial scales of patch structure for the application of remote habitat sensing as a surrogate for biological community structure.

2014
Wing, SR, Jack L, Shatova O, Leichter JJ, Barr D, Frew RD, Gault-Ringold M.  2014.  Seabirds and marine mammals redistribute bioavailable iron in the Southern Ocean. Marine Ecology Progress Series. 510:1-13.   10.3354/meps10923   AbstractWebsite

Biological vectors are important for redistribution of nutrients in many ecological systems. While availability of iron (Fe) to phyto plankton limits pelagic productivity in the Southern Ocean, biomagnification within marine food webs can lead to high concentrations of Fe in the diet of seabirds and marine mammals. We investigated patterns in concentrations of the micronutrients Fe, Co, Zn and Mn, and the toxins Cd and As, in the guano of oceanic, coastal and predatory seabirds and in faeces of 2 species of marine mammals that congregate to breed in the sub-Antarctic Auckland Islands. We found that much of the variability in concentrations of Fe, Co, Zn and Mn among species could be explained by foraging behaviour and by trophic position. We observed concentrations of Fe to be 8 orders of magnitude higher in the guano of predators and coastal foragers than in the sub-Antarctic mixed layer. High concentrations of As and Cd were associated with organic matter sources from macroalgae. Analyses of the molar ratio Fe:Al indicated that Fe within food webs supporting seabirds has likely been extensively recycled from its lithogenic source. Patterns in Fe:N among species indicated that Fe is concentrated 2 to 4 orders of magnitude in the guano of seabirds compared to limiting conditions for phytoplankton growth in sub-Antarctic waters. These data highlight the potential role of seabirds and marine mammals in the redistribution of micronutrients in the Southern Ocean and their likely role as key nutrient vectors in the ecosystem, particularly around the sub-Antarctic islands during the breeding season.

Leichter, JJ, Stokes MD, Vilchis LI, Fiechter J.  2014.  Regional synchrony of temperature variation and internal wave forcing along the Florida Keys reef tract. Journal of Geophysical Research-Oceans. 119:548-558.   10.1002/2013jc009371   AbstractWebsite

Analysis of 10 year temperature records collected along the Florida Keys reef tract (FLKRT) reveals strong, regional-scale synchrony in high-frequency temperature variation suggestive of internal wave forcing at predominately semidiurnal frequencies. In each year and at all sites, the amplitude of semidiurnal temperature variation was greatest from March to September, and markedly lower from October to February. Comparisons of the semidiurnal component of the temperature variation among sites suggest complex patterns in the arrival of internal waves, with highest cross correlation among closely spaced sites and synchrony in periods of enhanced internal wave activity across the length of the FLKRT, particularly in summer. The periods of enhanced semidiurnal temperature variation at the 20 and 30 m isobaths on the reef slopes appear to be associated with the dynamics of the Florida Current and the onshore movement of warm fronts preceding the passage of Florida Current frontal eddies. Regional-scale satellite altimetry observations suggest temporal linkages to sea surface height anomalies in the Loop Current (upstream of the Florida Current) and setup of the Tortugas Gyre. The synchronized forcing of cool water onto the reef slope sites across the FLKRT is likely to affect physiological responses to temperature variation in corals and other ectothermic organisms, as well as larval transport and nutrient dynamics with the potential for regionally coherent pulses of larvae and nutrients arriving on reef slopes across the FLKRT. Key Points Temperature variation from internal waves is modulated regionally in the FL Keys High-frequency temperature variation changes synchronously across sites Thermocline dynamics are associated with Florida Current frontal eddies

2013
Leichter, JJ, Aildredge AL, Bernardi G, Brooks AJ, Carlson CA, Carpenter RC, Edmunds PJ, Fewing MR, Hanson KM, Hench JL, Holbrook SJ, Nelson CE, Toonen RJ, Shburn LW, Wyatt ASJ.  2013.  Biological and physical interactions on a tropical island coral reef transport and retention processes on Moorea, French Polynesia. Oceanography. 26:52-63. AbstractWebsite

The Moorea Coral Reef Long Term Ecological Research project funded by the US National Science Foundation includes multidisciplinary studies of physical processes driving ecological dynamics across the fringing reef, back reef, and fore reef habitats of Moorea, French Polynesia. A network of oceanographic moorings and a variety of other approaches have been used to investigate the biological and biogeochemical aspects of water transport and retention processes in this system. There is evidence to support the hypothesis that a low-frequency counterclockwise flow around the island is superimposed on the relatively strong alongshore currents on each side of the island. Despite the rapid flow and flushing of the back reef, waters over the reef display chemical and biological characteristics distinct from those offshore. The patterns include higher nutrient and lower dissolved organic carbon concentrations, distinct microbial community compositions among habitats, and reef assemblages of zooplankton that exhibit migration behavior, suggesting multigenerational residence on the reef. Zooplankton consumption by planktivorous fish on the reef reflects both retention of reef-associated taxa and capture by the reef community of resources originating offshore. Coral recruitment and population genetics of reef fishes point to retention of larvae within the system and high recruitment levels from local adult populations. The combined results suggest that a broad suite of physical and biological processes contribute to high retention of externally derived and locally produced organic materials within this island coral reef system.

2012
Leichter, JJ, Stokes MD, Hench JL, Witting J, Washburn L.  2012.  The island-scale internal wave climate of Moorea, French Polynesia. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc007949   AbstractWebsite

Analysis of five-year records of temperatures and currents collected at Moorea reveal strong internal wave activity at predominantly semi-diurnal frequencies impacting reef slopes at depths >= 30 m around the entire island. Temperature changes of 1.5 degrees C to 3 degrees C are accompanied by surges of upward and onshore flow and vertical shear in onshore currents. Superimposed on annual temperature changes of approximately 3 degrees C, internal wave activity is high from Oct-May and markedly lower from Jun-Sep. The offshore pycnocline is broadly distributed with continuous stratification to at least 500 m depth, and a subsurface fluorescence maximum above the strong nutricline at approximately 200 m. Minimum buoyancy periods range from 4.8 to 6 min, with the maximum density gradient occurring at 50 to 60 m depth in summer and deepening to approximately 150 to 200 m in winter. The bottom slope angle around all of Moorea is super-critical relative to the vertical stratification angle suggesting that energy propagating into shallow water is only a portion of total incident internal wave energy. Vertical gradient Richardson numbers indicate dominance by density stability relative to current shear with relatively limited diapycnal mixing. Coherence and lagged cross-correlation of semi-diurnal temperature variation indicate complex patterns of inter-site arrival of internal waves and no clear coherence or lagged correlation relationships among island sides. Semi-diurnal and high frequency internal wave packets likely arrive on Moorea from a combination of local and distant sources and may have important impacts for nutrient and particle fluxes in deep reef environments.

Konotchick, T, Parnell PE, Dayton PK, Leichter JJ.  2012.  Vertical distribution of Macrocystis pyrifera nutrient exposure in southern California. Estuarine Coastal and Shelf Science. 106:85-92.   10.1016/j.ecss.2012.04.026   AbstractWebsite

We examined water column temperature time series profiles for several years at two locations in a single kelp (Macrocystis pyrifera) forest to characterize the alongshore variability of the nutrient climate that giant kelp is exposed to and compare it to the response of giant kelp. The differences in nutrient climate are due to differential alongshore vertical variations in temperature, a well-established proxy of nitrate, due to the topographically induced internal wave dynamics within the kelp forest. We observed the greatest temperature variability during summer and most of this variability occurred near the surface. The 14.5 degrees C isotherm, indicating the presence of nitrate, ranged the entire vertical extent of the water column, and was shallowest during the winter and in the southern portion of the kelp forest. Predicted water column integrated nitrate varies from 0 mu mol NO3-/m(2) to 431 mu mol NO3-/m(2) yielding a time series daily average of 0.12 gN/m(2)day (North La Jolla) and 0.18 gN/m(2)day (South La Jolla). Redfield conversion of these values puts the time series daily average for carbon production (upper limit) between 0.8 and 1.2 gC/m(2)day for the north and south parts of the bed respectively, and shows considerable variation at several time scales. Giant kelp in the southern portion of the forest exhibited greater stipe densities (a proxy for kelp production) than individuals in the northern portion, corresponding with the alongshore nutrient climate variability. The depth of the nutricline varied by up to 10 m over time scales as short as hours. Variability was greatest at diurnal and semi-diurnal frequencies, with shallower water column depths showing greatest variability. These depth-specific variations in temperature and nutrient exposure may have biologically important consequences for M. pyrifera especially during low nutrient seasons. (C) 2012 Elsevier Ltd. All rights reserved.

Ladah, LB, Filonov A, Lavin MF, Leichter JJ, Zertuche-Gonzalez JA, Perez-Mayorga DM.  2012.  Cross-shelf transport of sub-thermocline nitrate by the internal tide and rapid (3-6 h) incorporation by an inshore macroalga. Continental Shelf Research. 42:10-19.   10.1016/j.csr.2012.03.010   AbstractWebsite

During summer in shallow waters off Baja California, Mexico, the internal tide is a dominant thermal feature of the water column. However, its importance for sub-thermocline nutrient provision to benthic macroalgae is unknown. In order to determine if internal motions provide nutrients to macroalgae in summer, Ulva lactuca was outplanted at inshore stations for short (3 and 6 h) intervals, at the surface, 5 and 10 m depth, and tissue nitrogen content was measured before and after each deployment. Concurrently temperature, currents, and nutrients were measured using moored thermistors, current profilers, CTDs. Niskin bottles, and an in-situ UV absorbance nitrate sensor (ISUS). Discrete pulses of cool, nutrient-rich water were horizontally displaced at least 4 km on the shelf and shoaled more than 20 m depth at the semidiurnal frequency, resulting in more than a 10-fold change in the concentration of nitrate. Inshore, tissue nitrogen of Ulva outplants increased significantly during longer exposures to this cool water. At this site, the semidiurnal signal dominates water column temperature fluctuations from April to November, with summer showing the greatest cooling (up to 5 degrees C) in a one-hour period. We estimated that 11% of the days of a year show internal waves that would cause a significant change in nutrient availability to macroalgae at 5 m depth. This study supports the hypothesis that nitrate can reach and be rapidly incorporated by inshore macroalgae such as Ulva through transport forced by the internal tide, and that even very short (<1 h) nutrient pulses in nature are reflected in macroalgal tissue. We propose that at this site, the internal tide provides a significant, yet understudied, high frequency nutrient source to inshore primary producers, particularly in summer. (C) 2012 Elsevier Ltd. All rights reserved.

Wing, SR, McLeod RJ, Leichter JJ, Frew RD, Lamare MD.  2012.  Sea ice microbial production supports Ross Sea benthic communities: influence of a small but stable subsidy. Ecology. 93:314-323. AbstractWebsite

Diversity in guilds of primary producers enhances temporal stability in provision of organic matter to consumers. In the Antarctic ecosystem, where temporal variability in phytoplankton production is high, sea ice contains a diatom and microbial community (SIMCO) that represents a pool of organic matter that is seasonally more consistent, although of relatively small magnitude. The fate of organic material produced by SIMCO in Antarctica is largely unknown but may represent an important link between sea ice dynamics and secondary production in nearshore food webs. We used whole tissue and compound-specific stable isotope analysis of consumers to test whether the sea ice microbial community is an important source of organic matter supporting nearshore communities in the Ross Sea. We found distinct gradients in delta C-13 and delta N-15 of SIMCO corresponding to differences in inorganic carbon and nitrogen acquisition among sites with different sea ice extent and persistence. Mass balance analysis of a suite of consumers demonstrated large fluxes of SIMCO into the nearshore food web, ranging from 5% to 100% of organic matter supplied to benthic species, and 0-10% of organic matter to upper water column or pelagic inhabitants. A delta C-13 analysis of nine fatty acids including two key biomarkers for diatoms, eicosapentaenoic acid (EPA, 20:5 omega 3), and docosahexaenoic acid (DHA, 22:6 omega 3), confirmed these patterns. We observed clear patterns in delta C-13 of fatty acids that are enriched in C-13 for species that acquire a large fraction of their nutrition from SIMCO. These data demonstrate the key role of SIMCO in ecosystem functioning in Antarctica and strong linkages between sea ice extent and nearshore secondary productivity. While SIMCO provides a stabilizing subsidy of organic matter, changes to sea ice coverage associated with climate change would directly affect secondary production and stability of benthic food webs in Antarctica.

2011
Perez-Mayorga, DM, Ladah LB, Zertuche-Gonzalez JA, Leichter JJ, Filonov AB, Lavin MF.  2011.  Nitrogen uptake and growth by the opportunistic macroalga Ulva lactuca (Linnaeus) during the internal tide. Journal of Experimental Marine Biology and Ecology. 406:108-115.   10.1016/j.jembe.2011.05.028   AbstractWebsite

To determine if an opportunistic macroalga could increase its tissue nitrogen content in response to nutrient pulses forced by the internal tide, independent samples of the green alga Ulva lactuca (Linnaeus), previously cultured in nutrient-poor tanks, were outplanted in the nearshore ocean for 12 h deployment periods (8 daylight periods, 7 night periods), at three different depths (1.5. and 10 m), during stratified water column conditions and spring tides. Algal tissue nitrogen and growth weight (before and after each deployment), water-column temperature (continuous), and inorganic water-column nutrient concentrations (taken on 3 days during the experiment) were measured. Abrupt drops in temperature (up to 3 degrees C in an hour) occurred twice daily, at 10 m and occasionally at 5 m depth, associated with the internal tide. Date and deployment depth had significant effects on U. lactuca tissue nitrogen, showing a significant increase for deeper deployments and a positive correlation with the duration of cool water (< 15.8 degrees C, > 1 mu M NO(3)(-)) at the deployment depth. On days when temperature remained above 15.8 degrees C at all depths (15.8 degrees C was established as the temperature below which nitrate was above 1 mu M), no significant increase in tissue nitrogen was detected at any depth. Growth and tissue nitrogen were significantly and inversely correlated for mid-water (5 m) and bottom (10 m) deployments, but not at the surface. The data support the hypothesis that U. lactuca can increase tissue nitrogen in response to the cold phase of the semidiurnal internal tide. (C) 2011 Elsevier B.V. All rights reserved.

Stokes, MD, Leichter JJ, Wing S, Frew R.  2011.  Temperature variability and algal isotopic heterogeneity on a Floridian coral reef. Marine Ecology-an Evolutionary Perspective. 32:364-379.   10.1111/j.1439-0485.2011.00469.x   AbstractWebsite

On Conch Reef, Florida Keys, USA, we examine relationships between temperature variability measured with a unique, high-resolution sensor network, and the abundance of benthic reef organisms and carbon, nitrogen and oxygen stable isotopic composition of two abundant macroalgal species. Visualization of spatial temperature anomalies mapped onto measured reef bathymetry reveals episodic thermal patchiness and persistent thermal features likely indicating meter scale variability in nutrient supply. These patterns result from the relatively shallow depths of the offshore thermocline and an active internal wave field creating a hydrographic structure conducive to interactions of an offshore, subthermocline nutrient pool with reef topography. The distribution of major benthic reef organism taxa, as well as the stable isotopic composition of C, N and O, from the macroalgae Dictyota menstrualis and Halimeda tuna are all highly variable on scales of less than 10 m and are not necessarily depth-dependent. Covariability between spatial patterns of the temperature minima, depth and delta(13)C of D. menstrualis and H. tuna suggest a role of oceanographic forcing in inorganic carbon acquisition in these species. Comparisons of the measured temperature with the predicted temperature based on delta(18)O from H. tuna aragonite suggest a possible influence of low salinity water in the system and non-linear interactions between ambient conditions and calcification at fine temporal and spatial scales. These observations have implications for the interpretation of both contemporary and paleontologic reef environments where oceanographic forcing interacts with complex reef topography.

Denny, M, Dorgan KM, Evangelista D, Hettinger A, Leichter J, Ruder WC, Tuval I.  2011.  Anchor Ice and Benthic Disturbance in Shallow Antarctic Waters: Interspecific Variation in Initiation and Propagation of Ice Crystals. Biological Bulletin. 221:155-163. AbstractWebsite

Sea ice typically forms at the ocean's surface, but given a source of supercooled water, an unusual form of ice-anchor ice-can grow on objects in the water column or at the seafloor. For several decades, ecologists have considered anchor ice to be an important agent of disturbance in the shallow-water benthic communities of McMurdo Sound, Antarctica, and potentially elsewhere in polar seas. Divers have documented anchor ice in the McMurdo communities, and its presence coincides with reduced abundance of the sponge Homaxinella balfourensis, which provides habitat for a diverse assemblage of benthic organisms. However, the mechanism of this disturbance has not been explored. Here we show interspecific differences in anchor-ice formation and propagation characteristics for Antarctic benthic organisms. The sponges H. balfourensis and Suberites caminatus show increased incidence of formation and accelerated spread of ice crystals compared to urchins and sea stars. Anchor ice also forms readily on sediments, from which it can grow and adhere to organisms. Our results are consistent with, and provide a potential first step toward, an explanation for disturbance patterns observed in shallow polar benthic communities. Interspecific differences in ice formation raise questions about how surface tissue characteristics such as surface area, rugosity, and mucus coating affect ice formation on invertebrates.

McMurray, SE, Blum JE, Leichter JJ, Pawlik JR.  2011.  Bleaching of the giant barrel sponge Xestospongia muta in the Florida Keys. Limnology and Oceanography. 56:2243-2250.   10.4319/lo.2011.56.6.2243   AbstractWebsite

The giant barrel sponge Xestospongia muta is now the dominant habitat-forming organism on many Caribbean coral reefs and has been observed to undergo cycles of bleaching similar to those seen for reef-building corals. We examined bleaching of X. muta at 15-m, 20-m, and 30-m depths over 2000-2005 on Conch Reef, Key Largo, Florida, to determine the spatial and temporal patterns of bleaching, whether bleaching affected sponge mortality, and whether there was a relationship between bleaching and seawater temperatures. Bleaching increased significantly with depth and was greater in autumn than spring, but the response was not uniform across the sponge population. Bleaching of X. muta did not result in sponge mortality, corroborating the conclusion that cyanobacterial symbionts of the sponge provide little or no benefit to the host. There was greater sponge bleaching with higher minimum daily seawater temperature anomalies within 30 d of surveys in the spring. For both the spring and autumn, there was greater bleaching with a higher number of positive 0.5 degrees C daily anomalies within 14 d before surveys. Anomalously warm seawater temperatures fail to completely explain the variability of the observed bleaching response, however, and additional work would be required to conclusively determine whether a causal relationship between bleaching and temperature exists. As has been described for corals, bleaching of X. muta is likely a complex response that may be affected by multiple factors, including light availability and differences in bleaching susceptibility among Synechococcus symbionts.

Slattery, M, Lesser MP, Brazeau D, Stokes MD, Leichter JJ.  2011.  Connectivity and stability of mesophotic coral reefs. Journal of Experimental Marine Biology and Ecology. 408:32-41.   10.1016/j.jembe.2011.07.024   AbstractWebsite

Mesophotic coral ecosystems (MCEs), occurring at depths of 30 to over 150 m, represents the deep continuum of adjacent shallow coral reefs about which little is known. These reefs are most developed, and most frequently studied, in areas where light penetration is high and photosymbiotic reef-builders (corals) can persist as common members of the benthic community. The upper mesophotic zone typically extends to 60 m and comprises communities that are generally similar to those found in shallow reef systems. Below 60 m, the lower mesophotic zone is dominated by sponges and algae that are uncommon or absent from shallower areas, and a fish fauna that is largely specialized to these intermediate depths. It is likely that these ecosystems are much more widespread and important than was previously recognized. MCEs have the potential to function as refugia for shallow reefs because many disturbances in the upper 30 m may leave MCEs largely intact. Ontogenetic movement and spawning migrations by reef fishes provide an important ecological component to connectivity between shallow and mesophotic reefs and may represent the effective range of larval dispersal. However, there is currently limited information on genetic connectivity between deep and shallow coral reef populations, and this is a critical gap in understanding the role of MCEs in coral reef resilience. Here we review the connectivity and trophic relationships between the shallow and mesophotic coral reefs, the potential role of MCEs as refugia, and the effects of a new biological invader that exposes critical gaps in our understanding of the stability of these ecosystems. (C) 2011 Elsevier B.V. All rights reserved.

Haas, AF, Nelson CE, Kelly LW, Carlson CA, Rohwer F, Leichter JJ, Wyatt A, Smith JE.  2011.  Effects of Coral Reef Benthic Primary Producers on Dissolved Organic Carbon and Microbial Activity. Plos One. 6   10.1371/journal.pone.0027973   AbstractWebsite

Benthic primary producers in marine ecosystems may significantly alter biogeochemical cycling and microbial processes in their surrounding environment. To examine these interactions, we studied dissolved organic matter release by dominant benthic taxa and subsequent microbial remineralization in the lagoonal reefs of Moorea, French Polynesia. Rates of photosynthesis, respiration, and dissolved organic carbon (DOC) release were assessed for several common benthic reef organisms from the backreef habitat. We assessed microbial community response to dissolved exudates of each benthic producer by measuring bacterioplankton growth, respiration, and DOC drawdown in two-day dark dilution culture incubations. Experiments were conducted for six benthic producers: three species of macroalgae (each representing a different algal phylum: Turbinaria ornata - Ochrophyta; Amansia rhodantha - Rhodophyta; Halimeda opuntia - Chlorophyta), a mixed assemblage of turf algae, a species of crustose coralline algae (Hydrolithon reinboldii) and a dominant hermatypic coral (Porites lobata). Our results show that all five types of algae, but not the coral, exuded significant amounts of labile DOC into their surrounding environment. In general, primary producers with the highest rates of photosynthesis released the most DOC and yielded the greatest bacterioplankton growth; turf algae produced nearly twice as much DOC per unit surface area than the other benthic producers (14.0 +/- 2.8 mu mol h(-1) dm(-2)), stimulating rapid bacterioplankton growth (0.044 +/- 0.002 log10 cells h(-1)) and concomitant oxygen drawdown (0.16 +/- 0.05 mu mol L(-1) h(-1) dm(-2)). Our results demonstrate that benthic reef algae can release a significant fraction of their photosynthetically-fixed carbon as DOC, these release rates vary by species, and this DOC is available to and consumed by reef associated microbes. These data provide compelling evidence that benthic primary producers differentially influence reef microbial dynamics and biogeochemical parameters (i.e., DOC and oxygen availability, bacterial abundance and metabolism) in coral reef communities.

Omand, MM, Leichter JJ, Franks PJS, Guza RT, Lucas AJ, Feddersen F.  2011.  Physical and biological processes underlying the sudden surface appearance of a red tide in the nearshore. Limnology and Oceanography. 56:787-801.   10.4319/lo.2011.56.3.0787   AbstractWebsite

The sudden appearance at the surface of an alongshore-parallel band of red tide near Huntington Beach, California, is described in high spatial and temporal resolution using novel instrumentation including a global positioning system-tracked jet-ski. The scale of the surface chlorophyll a (Chl a) band was small (similar to 200 m cross-shore) and ephemeral (3 h) compared with the subsurface extent of the red tide (similar to 2 km, > 7 d). The red tide was dominated by the regionally common dinoflagellate Lingulodinium polyedrum (F. Stein) and had developed as a subsurface Chl a layer during the 7 d prior to the surface appearance. A few hours before the surface appearance, a subsurface patch of elevated Chl a (Chl a > 30 mu g L(-1)) was observed in 13-m total depth in the trough of a shoreward-propagating internal wave, consistent with dinoflagellate vertical swimming interacting with the internal wave-driven convergence. Internal wave-breaking-induced vertical mixing in similar to 8-m water depth vertically spread the Chl a patch to the surface, creating the alongshore surface band similar to 500 m from shore. Both the subsurface Chl a patch and the surface Chl a band were prevented from entering the surf-zone by a density barrier of warm water adjacent to the beach. These high-resolution observations emphasize the role of nearshore physical dynamics in controlling the duration and intensity of red tide exposure to coastal habitats.

Wing, SR, Leichter JJ.  2011.  Variation in environmental conditions in a subtidal prey refuge: effects of salinity stress, food availability and predation on mussels in a fjord system. Marine Ecology-Progress Series. 422:201-210.   10.3354/meps08911   AbstractWebsite

Prey refuges are fundamental structural features in communities. We investigated variability in environmental conditions within a subtidal prey refuge for the blue mussel Mytilus edulis galloprovincialis formed by the persistent low-salinity layer (LSL) in Doubtful Sound, New Zealand. Multi-year observations and fine-scale oceanographic surveys along the axis of Doubtful Sound show strong spatial gradients in salinity, temperature, chlorophyll a (chl a) and nitrate concentrations. Mean surface salinity ranged from similar to 5 in the inner fjord zone to 15 in the mid-fjord, and 25 to 30 in the entrance zone. A marked subsurface maximum in chl a was observed below the LSL at 3 to 7 m depth. Adult blue mussels were confined to the LSL with a sharp decline in abundance from the entrance to the inner regions of the fjord. In contrast, mussel recruitment was observed both within and below the LSL to 10 m depth, with highest recruitment in the mid-fjord zone at 6 m depth. To test whether patterns in growth and survival in the absence of predation were coincident with food supply and salinity stress, we transplanted mussels in predator exclusion cages at depths of 2, 4, 6, and 8 m within inner, mid-, and entrance fjord zones and measured growth over 213 d. Variation in salinity and temperature, rather than food availability, had the largest impact on mussel growth rates and explained 89% of the variability in mussel growth. Salinity alone explained 87% of the variability in mussel growth. Adult survival in the absence of predation did not significantly differ with depth or fjord zone. Environmental stress, primarily in the form of stress associated with low salinity, exerted the greatest influence on growth of mussels within this critical subtidal prey refuge.

2010
Edmunds, PJ, Leichter JJ, Adjeroud M.  2010.  Landscape-scale variation in coral recruitment in Moorea, French Polynesia. Marine Ecology-Progress Series. 414:75-89.   10.3354/meps08728   AbstractWebsite

In this study, we describe spatio-temporal variation in coral recruitment in the back reef of Moorea, French Polynesia, and explore the extent to which it is associated with community structure, seawater temperature, and wave regimes. Recruitment was assessed with settlement tiles deployed at 10 sites on the north, southeast, and southwest shores and sampled every 5 to 7 mo from 2005 to 2007. Temperature and waves were measured using thermistors and pressure sensors, and community structure was assessed using in situ surveys. Mean recruitment varied from 0 to 6 recruits tile(-1) (510 cm(2)) and was generally greater in the second compared to the first year of the study. Recruitment varied among sites, and this variation was similar between years but differed among sampling periods: acroporids were common on the southwest shore between January and September, but rare from September to January; poritids were common on the north shore between September and January, but rare on the southwest shore from January to September; and pocilloporids were found all year, but at slightly elevated densities between January and September. The mean daily seawater temperature, and the daily range, varied among sites, but neither was closely related to coral recruitment, and both were of biologically small magnitudes. The coral community structure also varied among sites, but likewise, was not associated with coral recruitment. In contrast, outer reef wave energy exposure, which likely played an important role in driving cross-reef transport of seawater into the lagoon, differed between shores and seasons in a pattern consistent with the variation in coral recruitment in the back reef. We hypothesize that coral recruitment in the back reef of Moorea is influenced by the interaction of seasonal variation in wave exposure, family-level differences in coral reproductive timing, and the spatial distribution of adult corals.