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Zirino, A, Elwany H, Facca C, Maicu F, Neira C, Mendoza G.  2016.  Nitrogen to phosphorus ratio in the Venice (Italy) Lagoon (2001-2010) and its relation to macroalgae. Marine Chemistry. 180:33-41.   10.1016/j.marchem.2016.01.002   AbstractWebsite

Analysis of the annually-averaged 2001-2010 monthly nutrient data from 13 stations in the Venice Lagoon (Italy) shows that the concentrations of dissolved nitrogen (N) species, measured as total dissolved N (TDN), have increased over time while that of phosphorus (P) species, measured as total dissolved P (TDP) have decreased. During the study period, the TDN/TDP ratio in the lagoon rose from about 46:1 to 100:1 (by atoms), a level at which the growth of benthic macroalgae is favored over that of sea grasses. The increase of the TDN/TDP ratio appears to be caused by two factors: (1) a small, but increasing amount of N in river water entering the lagoon, and (2) low P input combined with adsorption and entrapment of orthophosphate on colloidal iron oxides and carbonates at the water-sediment interface. This second mechanism would explain the increase in the TDN/TDP ratio, principally in zones of low salinity, where hydrodynamic residence times are long enough to permit N enrichment and result in macroalgal growth preferentially in the central, landward, side of the lagoon. However, an examination of the algal coverage of the lagoon floor from 2002 to 2010, indicates that while macroalgal abundance may be influenced by the N/P ratio, the spatial and temporal distribution during this period cannot be explained solely by this one feature. Nonetheless, this work points to the importance of considering the contributions that sediments in shallow lagoons make to the over-all system productivity and ecology and may be applicable to other shallow environments. (C) 2016 Elsevier B.V. All rights reserved.

Levin, LA, Neira C, Grosholz ED.  2006.  Invasive cordgrass modifies wetland trophic function. Ecology. 87:419-432.   10.1890/04-1752   AbstractWebsite

Vascular plants strongly control belowground environments in most ecosystems. Invasion by vascular plants in coastal wetlands, and by cordgrasses (Spartina spp.) in particular, are increasing in incidence globally, with dramatic ecosystem-level consequences. We examined the trophic consequences of' invasion by a Spartina hybrid (S. alterniflora X S. foliosa) in San Francisco Bay (USA) by documenting differences in biomass and trophic structure of benthic communities between sediments invaded by Spartina and uninvaded sediments. We found the invaded system shifted from all algae-bascd to a detritus-based food web. We then tested for a relationship between diet and tolerance to invasion, hypothesizing that species that consume Spartina detritus are more likely to inhabit invaded sediments than those that consume surface algae. Infaunal diets were initially examined with natural abundance stable isotope analyses and application of mixing models, but these yielded an ambiguous picture of food sources. Therefore, we conducted isotopic enrichment experiments by providing N-15-labeled Spartina detritus both on and below the sediment surface in areas that either contained Spartina or were unvegetated. Capitellid and nereid polychaetes, and oligochaetes, groups shown to persist following Spartina invasion of San Francisco Bay tidal flats, took up N-15 from labeled native and invasive Spartina detritus. In contrast, We found that amphipods, bivalves, and other taxa less tolerant to invasion consumed primarily surficial algae, based oil C-13 enrichment experiments. Habitat (Spartina vs. unvegetated patches) and location of' detritus (on or within sediments) did not affect N-15 uptake from cletritus. Our investigations support a "trophic shift" model for ecosystem response to wetland plant invasion and preview loss of key trophic support for fishes and migratory birds by shifting dominance to species not widely consumed by species at higher trophic levels.

Arntz, WE, Gallardo VA, Gutierrez D, Isla E, Levin LA, Mendo J, Neira C, Rowe GT, Tarazona J, Wolff M.  2006.  El NiƱo and similar perturbation effects on the benthos of the Humboldt, California, and Benguela Current upwelling ecosystems. Advances in Geosciences. 6:243-265.: European Geosciences Union, c/o E.O.S.T. 5, rue Rene Descartes Strasbourg Cedex 67084 France, [], [URL:] AbstractWebsite

To a certain degree, Eastern Boundary Current (EBC) ecosystems are similar: Cold bottom water from moderate depths, rich in nutrients, is transported to the euphotic zone by a combination of trade winds, Coriolis force and Ekman transport. The resultant high primary production fuels a rich secondary production in the upper pelagic and nearshore zones, but where O sub(2) exchange is restricted, it creates oxygen minimum zones (OMZs) at shelf and upper slope (Humboldt and Benguela Current) or slope depths (California Current). These hypoxic zones host a specifically adapted, small macro- and meiofauna together with giant sulphur bacteria that use nitrate to oxydise H sub(2)S. In all EBC, small polychaetes, large nematodes and other opportunistic benthic species have adapted to the hypoxic conditions and co-exist with sulphur bacteria, which seem to be particularly dominant off Peru and Chile. However, a massive reduction of macrobenthos occurs in the core of the OMZ. In the Humboldt Current area the OMZ ranges between <100 and about 600 m, with decreasing thickness in a poleward direction. The OMZ merges into better oxygenated zones towards the deep sea, where large cold-water mega- and macrofauna occupy a dominant role as in the nearshore strip. The Benguela Current OMZ has a similar upper limit but remains shallower. It also hosts giant sulphur bacteria but little is known about the benthic fauna. However, sulphur eruptions and intense hypoxia might preclude the coexistence of significant mega- und macrobenthos. Conversely, off North America the upper limit of the OMZ is considerably deeper (e.g., 500-600 m off California and Oregon), and the lower boundary may exceed 1000m. The properties described are valid for very cold and cold (La Nina and "normal") ENSO conditions with effective upwelling of nutrient-rich bottom water. During warm (El Nino) episodes, warm water masses of low oxygen concentration from oceanic and equatorial regions enter the upwelling zones, bringing a variety of (sub)tropical immigrants. The autochthonous benthic fauna emigrates to deeper water or poleward, or suffers mortality. However, some local macrofaunal species experience important population proliferations, presumably due to improved oxygenation (in the southern hemisphere), higher temperature tolerance, reduced competition or the capability to use different food. Both these negative and positive effects of el Nino influence local artisanal fisheries and the livelihood of coastal populations. In the Humboldt Current system the hypoxic seafloor at outer shelf depths receives important flushing from the equatorial zone, causing havoc on the sulphur bacteria mats and immediate recolonisation of the sediments by mega- and macrofauna. Conversely, off California, the intruding equatorial water masses appear to have lower oxygen than ambient waters, and may cause oxygen deficiency at upper slope depths. Effects of this change have not been studied in detail, although shrimp and other taxa appear to alter their distribution on the continental margin. Other properties and reactions of the two Pacific EBC benthic ecosystems to el Nino seem to differ, too, as does the overall impact of major episodes (e.g., 1982/1983(1984) vs. 1997/1998). The relation of the "Benguela Nino" to ENSO seems unclear although many Pacific- Atlantic ocean and atmosphere teleconnections have been described. Warm, low- oxygen equatorial water seems to be transported into the upwelling area by similar mechanisms as in the Pacific, but most major impacts on the eukaryotic biota obviously come from other, independent perturbations such as an extreme eutrophication of the sediments ensuing in sulphidic eruptions and toxic algal blooms. Similarities and differences of the Humboldt and California Current benthic ecosystems are discussed with particular reference to ENSO impacts since 1972/73. Where there are data available, the authors include the Benguela Current ecosystem as another important, non-Pacific EBC, which also suffers from the effects of hypoxia.

Neira, C, Levin LA, Grosholz ED.  2005.  Benthic macrofaunal communities of three sites in San Francisco Bay invaded by hybrid Spartina, with comparison to uninvaded habitats. Marine Ecology-Progress Series. 292:111-126.   10.3354/meps292111   AbstractWebsite

A hybrid cordgrass, formed from a cross between Spartina alterniflora (Atlantic cordgrass) and S. foliosa (Pacific cordgrass), has recently spread within the intertidal zone of south San Francisco Bay. Sediment properties and macroinfaunal community structure were compared in patches invaded by Spartina hybrid and adjacent uninvaded patches at 3 sites in San Francisco Bay (2 tidal flats and 1 Salicornia marsh). We hypothesized that (1) sediments vegetated by Spartina hybrid would have reduced sediment grain size, higher organic matter content, lower redox potential, lower salinity and reduced microalgal biomass relative to adjacent unvegetated tidal flat sediments, and (2) that differences in the sediment environment would correspond to changes in the infaunal invertebrate community structure and feeding modes. We observed 75 % lower total macro-faunal density and lower species richness in Spartina-vegetated sediments at Elsie Roemer (30 yr old invasion) than in an adjacent unvegetated tidal flat. This was due to lower densities of surface-feeding amphipods, bivalves, cirratulid and spionid polychaetes. The proportional representation of subsurface-deposit feeders was greater in Spartina patches than in unvegetated sediments. At a more recently invaded site (Roberts Landing; 15 yr invasion), Spartina patches differed from tidal flat sediments in composition, but not in abundance. Native (Salicornia) and Spartina patches exhibited similar sediment properties at San Mateo, where the Spartina hybrid invaded 8 to 10 yr earlier. No differences were detected in densities or proportions of surface- or subsurface-deposit feeders, but the proportion of carnivores/omnivores and grazers increased in the hybrid-invaded patches. These studies suggest that the invasive Spartina hybrid in south San Francisco Bay can have differing effects on sediment ecosystems, possibly depending on the location, age, or type of habitats involved.