Publications

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2009
Checkley Jr., DM, Bakun A, Barange M, Castro LR, Freon P, Guevara R, Herrick Jr. SF, McCall AD, Ommer R, Oozeki Y, Roy C, Shannon L, Van der Lingen CD.  2009.  Synthesis and perspective. Climate change and small pelagic fish. ( Checkley Jr. DM, Alheit J, Oozeki Y, Roy C, Eds.).:344-351., Cambridge, UK; New York: Cambridge University Press Abstract
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2008
Checkley, DM, Davis RE, Herman AW, Jackson GA, Beanlands B, Regier LA.  2008.  Assessing plankton and other particles in situ with the SOLOPC. Limnology and Oceanography. 53:2123-2136.   10.4319/lo.2008.53.5_part_2.2123   AbstractWebsite

We combined a Sounding Oceanographic Lagrangian Observer float with a Laser Optical Plankton Counter (LOPC) and a fluorometer to make an autonomous biological profiler, the SOLOPC. The instrument senses plankton and other particles over a size range of 100 mm to 1 cm in profiles to 300 m in depth and sends data ashore via satellite. Objects sensed by the LOPC include aggregates and zooplankton, the larger of which can be distinguished from one another by their transparency. We hypothesized that the diel production of particles and their loss by sinking and grazing are reflected in the change of the particle distribution. We present data from four deployments of the SOLOPC off California. Particle volume was maximal at the base of the surface mixed layer and correlated with chlorophyll a fluorescence. In a 3-d deployment in 2005, particle volume was greatest in the early evening and smallest in the morning, and average particle size increased with depth. Eigenvector analysis of the particle volume distribution as a function of diameter for each of the deployments yielded size peaks characteristic of planktonic crustaceans. Ship-based measurements showed that the abundance of opaque particles of 1.1-1.7 mm equivalent spherical diameter was positively correlated with copepods of this size and simultaneously collected in nets. This relationship was used with SOLOPC data to estimate the distribution of large copepods, which were most abundant beneath the depth of maximal particle flux, estimated from particle size and published sinking rates. Our data are consistent with a model with diel production of particles and their loss by sinking and grazing.

Rykaczewski, RR, Checkley DM.  2008.  Influence of ocean winds on the pelagic ecosystem in upwelling regions. Proceedings of the National Academy of Sciences of the United States of America. 105:1965-1970.   10.1073/pnas.0711777105   AbstractWebsite

Upwelling of nutrient-rich, subsurface water sustains high productivity in the ocean's eastern boundary currents. These ecosystems support a rate of fish harvest nearly 100 times the global mean and account for >20% of the world's marine fish catch. Environmental variability is thought to be the major cause of the decadal-scale biomass fluctuations characteristic of fish populations in these regions, but the mechanisms relating atmospheric physics to fish production remain unexplained. Two atmospheric conditions induce different types of, upwelling in these ecosystems: coastal, alongshore wind stress, resulting in rapid upwelling (with high vertical velocity, w); and wind-stress curl, resulting in slower upwelling (low w). We show that the level of wind-stress curl has increased and that production of Pacific sardine (Sardinops sagax) varies with wind-stress curl over the past six decades. The extent of isopycnal shoaling, nutricline depth, and chlorophyll concentration in the upper ocean also correlate positively with wind-stress curl. The size structure of plankton assemblages is related to the rate of wind-forced upwelling, and sardine feed efficiently on small plankters generated by slow upwelling. Upwelling rate is a fundamental determinant of the biological structure and production in coastal pelagic ecosystems, and future changes in the magnitude and spatial gradient of wind stress may have important and differing effects on these ecosystems. Understanding of the biological mechanisms relating fisheries production to environmental variability is essential for wise management of marine resources under a changing climate.

Takahashi, M, Checkley Jr. DM.  2008.  Growth and survival of Pacific sardine (Sardinops sagax) in the California Current region. Journal of the Northwest Atlantic Fisheries Society. 41:129-136. Abstract
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Reiss, CS, Checkley DM, Bograd SJ.  2008.  Remotely sensed spawning habitat of Pacific sardine (Sardinops sagax) and Northern anchovy (Engraulis mordax) within the California Current. Fisheries Oceanography. 17:126-136.   10.1111/j.1365-2419.2008.00469.x   AbstractWebsite

We use trivariate kernel density estimation to define spawning habitat of northern anchovy (Engraulis mordax) and Pacific sardine (Sardinops sagax) in the California Current using satellite data and in situ egg samples from the Continuous Underway Fish Egg Sampler (CUFES) deployed during surveys in April by the California Cooperative Oceanic Fisheries Investigations (CalCOFI). Observed egg distributions were compared with monthly composite satellite sea surface temperature (SST) and surface chlorophyll a (chl a) data. Based on the preferred spawning habitat, as defined in SST and chl a space, the satellite data were used to predict potential spawning habitat along two areas of the west coast of North America. Data from the southern area (21.5 to 39 degrees N) were compared to observations from the CUFES data for the period 1998-2005. Northern anchovy and Pacific sardine exhibited distinctly different spawning habitat distributions. A significant relationship was found between satellite-based spawning area and that measured during surveys for sardine. CUFES area estimated for sardine was similar in magnitude to that estimated from satellite data (similar to 60 000 km(2)). In contrast, spawning habitat of anchovy averaged between 1000 and 200 000 km(2) for the period 1998-2005, for CUFES and satellite estimates, respectively. Interannual variability in the area (km(2)) and duration (months) of estimates of suitable habitat varied between species and between the northern (39 to 50.5 degrees N) and southern portions of the California Current. Long-term monitoring of habitat variability using remote sensing data is possible in the southern portion of the California Current, and could be improved upon in the northern area with the addition of surveys better timed to describe relationships between observed and estimated spawning habitats.

2007
Curtis, KA, Checkley DM, Pepin P.  2007.  Predicting the vertical profiles of anchovy (Engraulis mordax) and sardine (Sardinops sagax) eggs in the California Current System. Fisheries Oceanography. 16:68-84.   10.1111/j.1365-2419.2006.00414.x   AbstractWebsite

Several published models exist for simulating vertical profiles of pelagic fish eggs, but no one has rigorously assessed their capacity to explain observed variability. In this study, we applied a steady-state model, with four different formulations for vertical diffusivity, to northern anchovy (Engraulis mordax) and Pacific sardine (Sardinops sagax) eggs in the California Current region. Vertical mixing profiles, based on wind speed and hydrography, were combined with estimated terminal ascent velocities of the eggs based on measurements of egg buoyancy and size, to simulate the vertical profiles of the eggs. We evaluated model performance with two data sets: (1) vertically stratified tows for both species and (2) paired samples for sardine eggs from 3-m depth and in vertically integrated tows. We used two criteria: whether the model predicted individual observed vertical profiles (1) as well as the observed mean and (2) better than the observed mean. Model predictions made with the formulation producing the most gradual profile of vertical diffusivity provided the best match to observations from both data sets and for both species. Addition of a random error term to the terminal ascent velocity further improved prediction for anchovy eggs, but not sardine. For the paired data, model prediction of integrated abundance from abundance at 3-m depth had significantly lower mean square error than prediction based on a linear regression of 3 m on integrated abundance. Our results support the feasibility of using data from the Continuous Underway Fish Egg Sampler quantitatively as well as qualitatively in stock assessments.

2006
Gonzalez-Quiros, R, Checkley DM.  2006.  Occurrence of fragile particles inferred from optical plankton counters used in situ and to analyze net samples collected simultaneously. Journal of Geophysical Research-Oceans. 111   10.1029/2005jc003084   AbstractWebsite

[1] We hypothesized that the optical plankton counter (OPC) senses particles in situ that are not collected by nets and analyzed in the laboratory. An OPC was deployed in situ between 1998 and 2004 in the mouth of a bongo net with 505-mu m-mesh nets in the upper 210 m at stations in the California Current Region. Here we compare paired data sets from the OPC in situ and the OPC analysis in the laboratory of the simultaneously collected net samples for four seasons of 2 years. We restricted our analysis to particle sizes 1.26 - 6.35 mm equivalent spherical diameter (ESD), a size class shown from the lab OPC data to be retained efficiently by the net. On average, 4 (3) times more particles by number (volume) were sensed in situ by the OPC than sensed in net collections by the OPC in the lab. These values varied an order of magnitude among the eight cruises examined. Time of day, distance offshore, season, year, chlorophyll a concentration, and Brunt-Vaisala frequency each explained significant variation in these differences. The excess of particles sensed in situ over that measured in the net samples was due primarily to smaller particles in the 1.26 - 6.35 mm ESD range. We infer that the particles measured by the OPC in situ but not in the lab were fragile and thus not collected by the net. We hypothesize that these fragile particles are primarily aggregates and abandoned houses of larvaceans.

2004
Jahncke, J, Checkley DM, Hunt GL.  2004.  Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation. Fisheries Oceanography. 13:208-223.   10.1111/j.1365-2419.2004.00283.x   AbstractWebsite

We hypothesized that change in the annual population size of guano-producing seabirds (cormorant, Phalacrocorax bougainvillii; booby, Sula variegata; pelican, Pelecanus thagus) is a response to changes in primary and secondary production of the Peruvian upwelling system. We tested this hypothesis by modeling nitrate input through upwelling to the upper layers of the ocean off Peru between 6degrees and 14degreesS using data on wind stress and sea surface temperature. The model predicted the amount of carbon fixed by primary production each year from 1925 to 2000, which was then apportioned to the Peruvian anchovy (Engraulis ringens) biomass and ultimately to the seabird population and the anchovy fishery, the largest single-species fishery on Earth. The model predicted a marked increase in primary production as a consequence of increasing wind stress. It overestimated the anchovy biomass after the collapse of the fishery in 1972, but closely predicted the growth of seabird populations from 1925 to the mid-1960s, and their decline thereafter, explaining about 94% of the variation in seabird numbers from 1925 to 2000. The model indicates the seabirds consumed 14.4% of the available anchovies and, thus, that seabirds consumed 2.3% of the new production, before the development of the anchovy fishery, and only 2.2% of the available anchovies and 0.3% of the new production after the development of the fishery. The model results clarify the roles that environmental and anthropogenic factors may have had in regulating the guano-producing seabird populations. It indicates that the growth of seabird populations from 1925 to 1955 was likely a response to increased productivity of the Peruvian upwelling system and that the subsequent drastic decline in seabird abundance was likely due to competition for food with the fishery, which caught similar to85% of the anchovies, which otherwise would have been available for the seabirds. This model also shows that an increase in oceanic primary production promotes reproductive success and population growth in higher trophic level organisms.

Johnson, CL, Checkley DM.  2004.  Vertical distribution of diapausing Calanus pacificus (Copepoda) and implications for transport in the California undercurrent. Progress in Oceanography. 62:1-13.   10.1016/j.pocean.2004.08.001   AbstractWebsite

Migration to deep water during diapause may contribute to the retention of several dominant oceanic calanoid copepod populations in eastern boundary current systems, where the mean flow of poleward undercurrents is in opposition to mean equatorward surface flow. The vertical distributions of Calanus pacificus late copepodid stages were measured at a 1200-m deep, open-ocean station in the Southern California Bight on 13 dates between April 2000 and March 2001 using a MOCNESS (multiple opening and closing net and environmental sensing system). Copepod vertical distribution was compared to the vertical position of the California Undercurrent. Diapausing C pacificus were primarily found between 300 and 400 m at the beginning of the diapause season, in June and July, and between 250 and 350 at the end of the diapause season, in November and January. Depth distributions were broader from August to October, ranging from about 350 m to the maximum depth sampled, 1100 m, and the median depth of diapausing C pacificus was deeper, up to 800-900 m, during this period. Maximal depths of diapausing C. pacificus, 1100-1000 m, were greater than have previously been reported. The mean depth of the California Undercurrent was 250 m, and its approximate depth range was 110-430 m. Diapausing C pacificus CV were abundant in the California Undercurrent at the beginning and end of the diapause season, in June to July and late-November to January, suggesting that poleward transport of diapausing copepods in the California Undercurrent contributes to C pacificus Population retention in the California Current System. (C) 2004 Elsevier Ltd. All rights reserved.

2003
Mullin, MM, Checkley DM, Thimgan MP.  2003.  Temporal and spatial variation in the sizes of California current macrozooplankton: analysis by optical plankton counter. Progress in Oceanography. 57:299-316.   10.1016/s0079-6611(03)00103-4   AbstractWebsite

Macrozooplankton in the southern California sector of the California Current had been reported to decrease from 1951 to 1998. We analyzed some of the same preserved samples of macrozooplankton taken in non-El Nino years 1955, 1956, 1966, 1981, 1984, 1991, 1995, and 1996, and also 1999, with the optical plankton counter (OPC) to determine whether all size categories changed similarly over time. The results from 1955 to 1996 could be interpreted either as a linearly decreasing trend (total biovolume decreased by 45%) or as a regime shift (decrease of 38% from pre- to post-1975 regimes). The largest zooplankters (>2.7 mm equivalent circular diameter, ecd) were relatively more important, in terms of biovolume, at night than by day, and offshore than onshore. Their biovolume decreased by the greatest relative amount, and biovolume of the smallest zooplankters (0.75-0.80 mm ecd) decreased hardly at all. The decrease in relative importance of large zooplankters was most evident in winter and spring, and was qualitatively similar by day and at night and in nearshore and offshore regions. Total biovolume increased in 1999 to the pre-1975 level, consistent with a possible shift to a new regime. Our results are consistent with (i) a change in biovolume of the large zooplankton over the sampled period; and (ii) a regime shift in the mid-1970s and, possibly, the late 1990s. The laboratory OPC is a useful instrument for the rapid and cost-effective analysis of preserved samples of zooplankton. (C) 2003 Elsevier Ltd. All rights reserved.

2001
Iwamoto, S, Checkley DM, Trivedi MM.  2001.  REFLICS: Real-time flow imaging and classification system. Machine Vision and Applications. 13:1-13.   10.1007/pl00013270   AbstractWebsite

An accurate analysis of a large dynamic system like our oceans requires spatially fine and temporally matched data collection methods. Current methods to estimate fish stock size from pelagic (marine) fish egg abundance by using ships to take point samples of fish eggs have large margins of error due to spatial and temporal undersampling. The real-time flow imaging and classification system (REFLICS) enhances fish egg sampling by obtaining continuous, accurate information on fish egg abundance as the ship cruises along in the area of interest. REFLICS images the dynamic flow with a progressive-scan area camera (60 frames/s) and a synchronized strobe in backlighting configuration. Digitization and processing occur on a dual-processor Pentium II PC and a pipeline-based image-processing board. REFLICS uses a segmentation algorithm to locate fish-egg-like objects in the image and then a classifier to determine fish egg, species, and development stage (age). We present an integrated system design of REFLICS and performance results. REFLICS can perform in real time (60 Hz), classify fish eggs with low false negative rates on real data collected from a cruise, and work in harsh conditions aboard ships at sea. REFLICS enables cost-effective, real-time assessment of pelagic fish eggs for research and management.

2000
Checkley, DM, Dotson RC, Griffith DA.  2000.  Continuous, underway sampling of eggs of Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax) in spring 1996 and 1997 off southern and central California. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 47:1139-1155. AbstractWebsite

Eggs of the northern anchovy and Pacific sardine were sampled at 3 m depth using the Continuous, Underway Fish Egg Sampler (CUFES) during two cruises off southern and central California in March and April 1996 and 1997. Simultaneous measurements were made of seawater temperature and salinity. Real-time AVHRR satellite images of sea surface temperature were processed onshore and transmitted to the ship at sea. Sardine and anchovy eggs were identified and counted live, at sea, and again ashore, preserved. A total of 2129 CUFES samples were collected during 41 d at sea and contained 62,409 sardine and 15,123 anchovy eggs. Sardine eggs were widespread and abundant in both cruises (mean concentrations: 5.2 and 5.1 eggs m(-3)) at and shoreward of the eastern boundary of the California Current. Anchovy eggs were found primarily inshore in the Southern California Eight and were less abundant (0.4 and 1.0 eggs m(-3), respectively, for the two cruises). Temperature-salinity plots indicated distinct separation of the spawning habitat of these two species and consistency in habitat between cruises in successive years. The distributions of sardine eggs and euphausiids collected by CUFES were complementary in space and time. (C) 2000 Elsevier Science Ltd. All rights reserved.

Checkley Jr., DM.  2000.  Michael M. Mullen: A biography. PICES Press. 8:13-17. Abstract
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1999
Checkley Jr., DM, Ortner PB, Werner FE, Settle LR, Cummings SR.  1999.  Spawning habitat of the Atlantic menhaden in Onslow Bay, North Carolina. Fisheries Oceanography. 8:22-36.: Blackwell Science Ltd   10.1046/j.1365-2419.1999.00019.x   AbstractWebsite

The Continuous, Underway Fish Egg Sampler (CUFES) was used to sample pelagic eggs of the Atlantic menhaden (Brevoortia tyrannus) from 3-m depth off North Carolina in winter 1993–94 and 1994–95. Simultaneous measurements were made of temperature, salinity, and the concentration of chlorophyll a. The maximal concentration of eggs was 346 eggs m−3. Eggs were highly aggregated in patches which occurred between the Gulf Stream and mid-shelf fronts (17–23°C, 36.0–36.4‰). Unexpectedly, eggs were found almost exclusively in water of 20–60 m (mode 20 m) bottom depth. Thus, spawning appears related to bathymetry as well as hydrography. Variograms for egg concentration indicated a mean (± SE) patch scale of 3.6 ± 1.7 km and a high degree of spatial variance explained by CUFES sampling. Lagrangian modelling of particles moving in response to tides, winds, and a prescribed flow from the north indicated that the region of observed, maximal occurrence of eggs is favourable for the retention of eggs and larvae on the shelf adjacent to inlets used to enter nursery areas.

1998
Cambalik, JJ, Checkley DM, Kamykowski D.  1998.  A new method to measure the terminal velocity of small particles: A demonstration using ascending eggs of the Atlantic menhaden (Brevoortia tyrannus). Limnology and Oceanography. 43:1722-1727. AbstractWebsite

A new method, incorporating video, motion analysis, and a novel experimental apparatus, was used to measure the terminal velocity of particles. The method facilitated the investigation of treatment effects and maximized the number of measurements for each replicate, thus improving the statistics for a population of particles. The eggs of the Atlantic menhaden (Brevoortia tyrannus) were used to demonstrate the method by investigating the effects of salinity and stage of development on their ascent rate. Egg ascent rate was greatest at intermediate salinity (36.5 parts per thousand) and decreased in the late stage of embryonic development. We estimate eggs at oceanic salinities (>35.5 parts per thousand) in nature to ascend at 0.19-0.25 cm s(-1).

Van der Lingen, CD, Checkley D, Barange M, Hutchings L, Osgood K.  1998.  Assessing the abundance and distribution of eggs of sardine, Sardinops sagax, and round herring, Etrumeus whiteheadi, on the western Agulhas Bank, South Africa, using a continuous, underway fish egg sampler. Fisheries Oceanography. 7:35-47. AbstractWebsite

A continuous, underway fish egg sampler (CUFES) was employed to assess the abundance and distribution of eggs of both sardine, Sardinops sagax, and round herring, Etrumeus whiteheadi, on the Western Agulhas Bank, South Africa, during September 1996. Samples were collected while underway along six inshore/offshore transects, and at stations along the transects. Volumetric estimates of egg density (eggs m(-3)) from on-station CUFES samples were highly correlated with both volumetric and areal (eggs m(-2)) estimates of egg density from samples collected from CalVET net hauls at these stations, demonstrating the validity of this novel sampling technique. Sardine and round herring eggs were encountered in a band running parallel to the coast and extending from 10 to 33 nautical miles offshore to the shelf edge, and highest egg densities were associated with strong north-west-flowing currents in the region of the shelf edge. Collecting samples while underway increased the precision of the estimate of mean egg density for sardine eggs but not for round herring eggs. The use of CUFES in obtaining a fine-scale resolution of sardine egg distribution, and as a tool for stock assessment, are discussed.

Iwamoto, I, Trivedi MM, Checkley Jr. DM.  1998.  Real-time detection and classification of objects in flowing water. Machine vision systems for inspection and metrology VII : 4-5 November, 1998, Boston, Massachusetts . 3521( Batchelor BG, Miller JWV, Solomon S, Eds.).:214-220., Bellingham, Wash.: SPIE (International Society for Optical Engineering) Abstract
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1997
Osgood, KE, Checkley DM.  1997.  Seasonal variations in a deep aggregation of Calanus pacificus in the Santa Barbara Basin. Marine Ecology-Progress Series. 148:59-69.   10.3354/meps148059   AbstractWebsite

A deep aggregation of fifth copepodid (C5) Calanus pacificus in the Santa Barbara Basin (SBB) was mapped over 1 1/2 yr with zooplankton net tows, an optical plankton counter, and a moored acoustic Doppler current profiler. High concentrations of diapausing C5 C. pacificus built up in the deep waters of the SBB during the summer and into the fall. During the buildup, the deep aggregation moved up from the bottom as oxygen became depleted in the basin's deep waters. The deep aggregation apparently builds up due to the basin trapping C5s that migrate below the sill depth from water advected over the basin. C5s are retained within the basin until they swim, or are forced, above the sill depth. Possible mechanisms responsible for the dispersal of the deep aggregation are migration to the surface waters with subsequent dispersal by surface currents, advection over the basin's sill due to the buildup of oxygen-deficient water, or flushing of the basin's deep water.

Checkley, DM, Ortner PB, Settle LR, Cummings SR.  1997.  A continuous, underway fish egg sampler. Fisheries Oceanography. 6:58-73.   10.1046/j.1365-2419.1997.00030.x   AbstractWebsite

We describe a method to sample the highly contagious distribution of pelagic fish eggs. CUFES, the continuous, underway fish egg sampler, consists of a submersible pump, concentrator, electronics and sample collector. This system operates continuously and under nearly all sea conditions, providing a real-time estimate of the volumetric abundance of pelagic fish eggs at pump depth, usually 3 m. CUFES-derived estimates of volumetric abundance agree well with those from nets towed at pump depth and with areal abundance estimated from vertically integrated plankton tows. CUFES has been used successfully to sample the eggs of menhaden, pinfish, sardine, and anchovy off the coasts of the eastern and western United States and South Africa. Two large patches of eggs of the Atlantic menhaden were sampled off North Carolina in winter 1993-94, had a linear scale of 5-10 km, and were found in waters between the Gulf Stream and mid-shelf front. Spawning location may he related to bathymetry. CUFES is now being used to estimate spawner biomass by the daily egg production method. An optical plankton counter provided accurate estimates of the number of Atlantic menhaden eggs sample by CUFES.

Osgood, KE, Checkley DM.  1997.  Observations of a deep aggregation of Calanus pacificus in the Santa Barbara Basin. Limnology and Oceanography. 42:997-1001. AbstractWebsite

An optical plankton counter/CTD package was used with zooplankton net samples to map the distribution of fifth copepodid (C5) Calanus pacificus in the Santa Barbara Basin region during two autumn cruises. Diapausing C5 C. pacificus were aggregated in a layer just above the basin's oxygen-deficient bottom waters and below its sill depth. The maximal concentration measured was 6,900 ind. m(-3) from a net sample spanning a depth range double the thickness of the C5 layer Although the C5 concentration varied, the layer was found at all stations of sufficient bottom depth within the basin. During November 1994, C5 C. pacificus accounted for 95-97% of all zooplankton caught in net samples from the layer. Relatively low concentrations of deep-dwelling C5 C. pacificus were observed at nearby stations outside the basin. We hypothesize that C5 C. pacificus descend into the Santa Barbara Basin at diapause, are trapped, and accumulate in a region of relatively low predator abundance. The resultant aggregation is estimated to contain a significant fraction of the regional C. pacificus population and thus assumes an important role in its dynamics.

1996
Osgood, KE, Checkley Jr. DM.  1996.  Concentration of Calanus pacificus in the Santa Barbara Basin. EOS Trans. AGU. 76:36. Abstract
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Checkley Jr., DM, Cooper T, Lennert C.  1996.  Plankton pattern within and below the surface mixed layer. EOS Trans. AGU. 76:198. Abstract
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1992
Checkley, DM, Uye S, Dagg MJ, Mullin MM, Omori M, Onbe T, Zhu MY.  1992.  Diel variation of the zooplankton and its environment at neritic stations in the Inland Sea of Japan and the north-west Gulf of Mexico. Journal of Plankton Research. 14:1-40.   10.1093/plankt/14.1.1   AbstractWebsite

Diel variations in the zooplankton and its environment were investigated at two, contrasting neritic stations. The first (BG-1), in the Inland Sea of Japan, was mixed and eutrophic, while the second (GM-1), in the north-west Gulf of Mexico, was stratified and oligotrophic. Intensive studies were conducted at each station in late summer for 2-3 days. Dissolved nutrients and the particulate matter were evenly distributed in time and space at BG-1, but were variable, and often maximal at depth in a nepheloid layer, at GM-1. For each station, approximately 20 categories of zooplankton were enumerated in samples collected with a plankton pump and retained on approximately 100-mu-m mesh filters, In general, the zooplankton at BG-1 exhibited little diel variation in abundance and distribution. By contrast, most types of zooplankton at GM-1 performed diel vertical migrations, though primarily within the lower half of the water column between the thermocline and nepheloid layer. Significantly, similar taxa and stages did not always behave similarly in these two, differing environments, nor did the zooplankton at GM-1 tend to aggregate at the depths of maximal particle abundance or primary productivity. We suggest that studies of diel variation of the distribution and abundance of the zooplankton often require more intense sampling, in time and space, in environments which are stratified rather than mixed.

Checkley, DM, Dagg MJ, Uye S.  1992.  Feeding, excretion and egg production by individuals and populations of the marine, planktonic copepods, Acartia spp. and Centropages furcatus. Journal of Plankton Research. 14:71-96.   10.1093/plankt/14.1.71   AbstractWebsite

Diel variations in vertical distribution, gut pigment content, ammonium excretion and egg production were investigated for adult females of Acartia erythraea and A. pacifica in the vertically mixed Inland Sea of Japan and Centropages furcatus in the stratified, neritic Gulf of Mexico. Gut pigment content and egg production rate were maximal at night and ammonium excretion was maximal during the daytime. Neither A. erythraea nor A. pacifica adult females showed an apparent diel migration, but the former were highly concentrated in the sur-face layer during the afternoon. In contrast, C. furcatus adult females showed a clear diel migration, residing immediately above the bottom during the daytime and being concentrated between 10 and 25 m depth during the night-time. Individual-based data on gut content and excretion and egg production rates were combined with vertical-distribution data to calculate population values. In the Inland Sea of Japan, the resultant pattern for Acartia spp. reflected the diel variation in physiological rates and even distribution of adult females, except for the afternoon, surface aggregation of A. erythraea. In the Gulf of Mexico, the pattern for C. furcatus reflected largely the diel variation in each rate process and the heterogeneous distribution of adult females in the water column. Elevated nocturnal feeding activity of these copepods may be due to an endogenous rhythm. The daytime maximum in ammonium excretion and night-time maximum in egg production rate indicated approximate half-day and day time lags, respectively, after the intake of food until its conversion into dissolved excreta and released eggs.