Gut Pigment Research Articles

Terrestrial carbon contribution to lake food webs: could the classical stable isotope approach be misleading?

Carbon stable isotope analyses have been widely used to estimate terrestrial carbon contribution to lake secondary production. In such approaches, phytoplankton is tacitly assumed as a single, isotopically homogenous source. Such assumption might be valid if (i) zooplankton do not feed selectively on specific algal taxa within bulk phytoplankton, or (ii) although zooplankton do feed selectively, the variability in the δ13C values amongst the different algal taxa is small compared with the variability between the δ13C values of bulk phytoplankton and terrestrial end-members. In a summer field study of six coastal lakes of British Columbia, Canada, we tested these assumptions using gut pigments and stable isotope measures on zooplankton and particulate organic matter. Results revealed that filter-feeding cladocerans positively selected cryptophytes, shown to be substantially 13C-enriched compared with bulk phytoplankton and even with the terrestrial end-member. Comparing a classical two-source mixing model and a model that accounted for algal isotopic heterogeneity, we showed that the use of a classical two-source model can result in an overestimation of terrestrial contribution to zooplankton secondary production.

Impacts of ontogenetically migrating copepods on downward carbon flux in the western subarctic Pacific Ocean

Abstract To evaluate the impacts of ontogenetically (seasonally) migrating copepods on carbon transport to the mesopelagic zone, we investigated depth distribution, population structure, and feeding activity of the ontogentic copepod community in the western subarctic Pacific Ocean from day–night pairs of zooplankton samples down to 1000 m during the VERtical Transport In the Global Ocean (VERTIGO) program. Over the 31 July–16 August 2005 study period, the biomass of Neocalanus cristatus and Neocalanus plumchrus predominated in the near surface waters, while Neocalanus flemingeri was already dormant at depth. We observed a strong diel migration for Metridia pacifica, and a seasonal downward migration for Eucalanus bungii. Based on gut pigment analysis, ingestion rate of the copepod community was 214–375 mg C m−2 day−1, which was equal to 26–37% of the concurrent primary production. However, comparison of grazing estimated from gut pigments to calculated carbon demand of the copepod community indicates that phytoplankton comprised 37–59% of the ingested carbon. Thus, the copepod community appears to have also relied on detritus and microzooplankton for their nutrition, likely because primary production during this time was dominated by picophytoplankton too small to be grazed by these large copepods. Fecal pellet flux by the copepod community was estimated to account for 141–223% of the sedimentary particulate organic carbon (POC) flux at 150 m, suggesting considerable fragmentation and consumption of pellets in the upper layers. Fecal pellets alone were adequate to meet copepod carbon demand in the surface 0–150 m layer. Active carbon flux by diel migration of M. pacifica (respiration, egestion, and mortality) was 4–17 mg C m−2 day−1, equal to 6–44% of sedimentary POC flux at 150 m. Active carbon flux by N. flemingeri ontogenetic migration (i.e., respiration and mortality at depth) contributed 246 mg C m−2 year−1, equal to 9% of sedimentary POC flux at 1000 m. The imminent downward migration of N. cristatus and N. plumchrus would lead to an additional ontogenetic carbon flux on the order of 1719 mg C m−2 year−1. Copepod fecal pellet transport and active transport by diel and ontogenetic migration are thus important carbon fluxes during a season dominated by small phytoplankton, and ontogenetic migrants in the subarctic Pacific Ocean play a relatively more important role in active carbon flux compared with other open-ocean regions.

Zooplankton community structure, biomass and role in carbon fluxes during the second half of a phytoplankton bloom in the eastern sector of the Kerguelen Shelf (January–February 2005)

During the KEOPS survey, zooplankton was sampled with vertical tows to estimate zooplankton stock and to study its composition and size structure both using traditional taxonomic identification and Optical Plankton Counter (OPC). Mesozooplankton OPC-biomass derived from OPC size spectra and integrated over 200 m was variable with average values about 10 g C m−2 along transects A and B and at the fixed station KERFIX, and only ∼5 g C m−2 along transect C. Stations in the most oceanic area (A11, B11, and C11) presented biomass values 3 times lower than the mean value of their respective transects, highlighting a clear decrease of the biomass beyond the shelf. The mesozooplankton community was dominated by copepods, particularly by large- and medium-size calanoids and small Oithonidae. Large numbers of different copepodites stages and nauplii were found, as well as exuviae, indicating that individuals were in active growing phase over the whole area. Euphausiids, chaetognaths, appendicularians, amphipods, polychaetes, ostracods and salps were found as well. Two reference stations, A3 located in the middle of the bloom on the shelf and C11 in the oceanic waters, were visited several times during the cruise. No particular temporal variations, neither in biomass nor in community structure, were observed, but differences in integrated biomass (average biomass at A3: 10.6 g C m−2; at C11: 2.8 g C m−2) between oceanic and shelf stations clearly show an enhanced secondary production on the shelf. Additional measurements at some stations were performed in order to quantify ingestion (gut contents) and respiration rates on key species and size groups. Gut pigment contents were higher during the first half of the survey at both stations, showing clear temporal changes probably linked to the prey field, with lower values always reported in the oceanic waters compared to the shelf. Values of respiration and ingestion rates extrapolated from OPC size spectra using published allometric relationships are discussed.

Measurement of copepod predation on nauplii using qPCR of the cytochrome oxidase I gene

A method to directly measure predation rates by older stage copepods upon copepod nauplii using species-specific primers for the mitochondrial cytochrome oxidase subunit one gene (mtCOI) and real-time quantitative PCR (qPCR) was developed. The general approach is to determine the mtCOI gene copy number of an individual prey organism and the copy number of the same gene in the stomachs of predatory copepods collected in the field. From the knowledge of DNA disappearance rates in the stomachs, ingestion rates can be calculated. In October 2006, laboratory experiments were carried out with Acartia tonsa N1 and N2 as prey and adult female Centropages typicus as predator. The copepods were collected in Narragansett Bay, USA. A. tonsa mtCOI copy numbers copepod−1 were determined for stages N1–C1 and for adults. A. tonsa DNA was detectable in the guts of the predators for as long as 3 h. Exponential rates of decline in prey DNA from the stomachs of the predators are similar to those measured for gut pigments. Because of the very small amount of DNA in an individual N1 or N2 nauplius, procedures were developed to maximize the quantitative extraction and recovery of DNA and to increase the sensitivity of the method. Two quite divergent haplotypes of A. tonsa were found in Narragansett Bay, which required separate qPCR primers; one was present in summer (July) and the other in fall and winter (October and February). With modification, the methods in this study can likely be applied to a range of predator–prey systems.

Quantitative PCR to estimate copepod feeding

Copepods play a central role in marine food webs as grazers of plankton and as key prey for many predators. Therefore, quantifying their specific trophic interactions is critical for understanding the role of copepods in ocean processes. However, because of methodological constraints, it remains difficult to investigate in situ copepod feeding without reliance on laborious intrusive and potentially biased incubation approaches. Recent advances in PCR-based methodologies have demonstrated the feasibility of directly identifying copepod diets based on prey DNA sequences. Yet, obtaining quantitative information from these approaches remains challenging. This study presents results of systematic efforts to develop a quantitative PCR (qPCR) assay targeted to 18S rRNA gene fragments to estimate copepod gut content of specific species of prey algae. These results were first compared to gut content estimates based on fluorescence in the copepod Calanus finmarchicus fed monocultures of two different microalgae species in controlled laboratory studies. In subsequent field studies, we compared feeding rates obtained by microscopy and qPCR for Temora longicornis and Acartia clausi feeding on the haptophyte Phaeocystis globosa in natural blooms. These investigations demonstrate a semi-quantitative relationship between gut content estimates derived from qPCR, gut pigment, and direct microscopy. However, absolute estimates of gut content based on qPCR methodology were consistently lower than expected. This did not appear to be explained by the extraction methods used, or interference by non-target (predator) DNA in the PCR reactions, instead suggesting digestion of prey-specific nucleic acids. Furthermore, the 18S rDNA target gene copy number of the phytoplankton varied with growth phase. Nonetheless, when prey target gene copy number in the ambient water is quantified, the qPCR-approach can be compared to other methods, and then used to semi-quantitatively estimate relative copepod grazing on specific prey in situ without involving further incubations. A distinct advantage of a DNA-based molecular approach compared to gut fluorescence and direct microscopic observation, is the ability to detect non-pigmented and macerated prey. Future studies should aim to correct for breakdown in prey DNA and perform extensive calibrations to other methods in order to achieve a quantitative measure of feeding rates in situ.

Trophic relationships in deep-water decapods of Le Danois bank (Cantabrian Sea, NE Atlantic): Trends related with depth and seasonal changes in food quality and availability

The trophic relationships of decapod crustaceans on Le Danois bank (NE of Iberian Peninsula, NE Atlantic Ocean) were studied within the framework of the multidisciplinary project ECOMARG during two surveys, one in October 2003 and the other in April 2004. The diets of eleven species of decapods were analyzed and, within a rather continuous gradient of food source exploitation, 3 trophic groups were identified: (1) plankton feeders, comprising the shrimps Acanthephyra pelagica, Sergia robusta, and Pasiphaea tarda, which preyed on meso-bathypelagic taxa such as euphausiids and calanoids; (2) benthos feeders, comprising the crangonids Pontophilus norvegicus and Pontophilus spinosus, the crab Geryon trispinosus and the shrimp Aristeus antennatus; and (3) an intermediate group, including the rest of species, with mixed diets that included detritus. Among the third group, anomurans ( Munida tenuimana, Pagurus alatus, and Parapagurus pilosimanus) consumed phytoplanktonic detritus in April, suggesting a link with peaks of surface Chl a occurring between March and April in the study area. Gut pigment and isotopic (δ 13C/δ 15N correlations) analyses revealed that assemblages inhabiting the top of the bank (455–612 m) and the inner basin (642–1048 m, close to the Lastres canyon head) had different food sources, with species inhabiting the deepest region exhibiting a stronger dependence on marine snow derivatives. These results are consistent with the higher proportion of mud and sediment organic matter (OM) content in the inner basin (82.2% pellites; 6.3% OM at 1028 m) compared to the top of the Le Danois bank (only 13.9% pellites; 2.8% OM at 485 m), which is a hydrodynamically more active zone. Exploitation of different food sources is also consistent with differences in the trophic level of species, inferred from stable δ 15N isotope analyses, which yield values ranging from 6.88‰ for the hermit crab P. alatus to 13.52‰ for the crangonid shrimp P. norvegicus. Stomach fullness was higher in April 2004 than in October 2003, both between and within species of the dominant decapods, including detritus feeders ( M. tenuimana) and benthos feeders (e.g. G. trispinosus, P. norvegicus). Most species exhibited a parallel increase in their density in April 2004, with a significant positive correlation between density and stomach fullness. This increase coincides with a peak of surface Chl a concentration occurring in March–April.

Reassessment of the gut pigment method for estimating in situ zooplankton ingestion

Correction for chlorophyll pigment destruction has been frequently used in calculation of copepod ingestion rates using the gut pigment method. We argue that tracers in the gut may be either digested and assimilated, or evacuated, and that both processes are taken into account when an evacuation (disappearance) rate curve is determined. As a result, any correction for gut pigment destruction in calculating ingestion rate is inappropriate.

Grazing by Pyrosoma atlanticum (Tunicata, Thaliacea) in the south Indian Ocean

Pyrosomas are colonial tunicates capable of forming dense aggregations. Their trophic function in the ocean, as well as their ecology and physiology in general, are extremely poorly known. During the ANTARES-4 survey (January and February 1999) their feeding dynamics were investigated in the south Indian Ocean. Results show that their in situ clearance rates may be among the highest recorded in any pelagic grazer, with up to 35 l h -1 per colony (length: 17.9 ± 4.3 (SD) cm). Gut pigment destruction rates, estimated for the first time in this tunicate group, are higher than those previously measured in salps and appendiculari- ans, ranging from 54 to virtually 100% (mean: 79.7 ± 19.8%) of total pigment ingested. Although individual colony ingestion rates were high (39.6 ± 17.3 (SD) µg pigment d -1 ), the total impact on the phytoplankton biomass and production in the Agulhas Front was rela- tively low, 0.01 to 4.91% and 0.02 to 5.74% respec- tively, as a result of the low abundance of colonies. Colonies showed higher retention efficiency for parti- cles larger than 10 µm. Low levels of lipids and per- centages of triacylglycerols and free fatty acids were found in zooids. Markers for diatoms (C16 PUFA), dinoflagellates (22:6n-3, 18:5n-3) and prymnesiophytes (18:1n-9, 18:4n-3) were observed. Stepwise discrimi- nant analysis of published data on phytoplankton fatty acid showed a strong similarity between the composi- tion of the 2 neutral lipid classes found in P. atlanticum and that of both dinoflagellates and prymnesiophytes. The sterol composition confirmed this result with the contribution of 24-methylcholest-5,22-dien-3β-ol, cho- lesterol, 24-methylenecholesterol and 24-ethylcholes- terol. Colonies exhibited large numbers of ciliate pro- tozoans (possibly Strombidium sp.). It is not clear whether this constituted a mere opportunistic intru- sion, or rather a more stable association between the 2

Functional responses of copepod nauplii using a high efficiency gut fluorescence technique

To investigate copepod nauplii ingestion rates on phytoplankton, we have adapted the traditional gut fluorescence technique as it can be used with lower gut pigment concentrations. With the improved technique, laboratory experiments were performed to estimate functional responses for nauplii of Calanus helgolandicus and Centropages typicus. Nauplii were raised from eggs to copepodites and the experiments were performed with stages NIV-NV. Gut evacuation rates and ingestion rates were measured on Isochrysis galbana at different concentrations. Specific ingestion rates ranged between 0.038–0.244 μg C μg−1 nauplii C d−1 for C. typicus and 0.041–1.412 μg C μg−1 nauplii C d−1 for C. helgolandicus. Both species showed a type III functional response, reaching a saturation concentration at around 600 μgC l−1 for C. typicus and 800 μgC l−1 for C. helgolandicus.

Periodic swarms of the salp Salpa aspera in the Slope Water off the NE United States: Biovolume, vertical migration, grazing, and vertical flux

Sampling during four summers over a twenty-seven year period has documented dense populations of Salpa aspera in the Slope Water south of New England, northeastern United States. The salps demonstrated a strong pattern of diel vertical migration, moving to depth (mostly 600–800 m) during the day and aggregating in the epipelagic ( < 100 m ) at night. Filtration rates determined from both gut pigment analysis and direct feeding experiments indicated that both the aggregate and solitary stages filtered water at rates ranging from 0.5 to 6 l h - 1 ml - 1 biovolume. Maximum displacement volumes of salps measured were 5.7 l m - 2 in 1986 and 1.6 l m - 2 in 1993. Depending on the year, the sampled salp populations were calculated to clear between 8 and 74% of the upper 50 m during each 8 h night. Total fecal output for the same populations was estimated to be between 5 and 91 mg C m - 2 night - 1 . These results, and other observations, suggest this region is a salp “hot spot”, with swarms of S. aspera developing seasonally on a frequent basis.

Diurnal gut pigment rhythm and metabolic rate of Calanus euxinus in the Black Sea

The vertical distribution, diel gut pigment content and oxygen consumption of Calanus euxinus were studied in April and September 1995 in the Black Sea. Gut pigment content of C. euxinus females was associated with diel vertical migration of the individuals, and it varied with depth and time. Highest gut pigment content was observed during the nighttime, when females were in the chlorophyll a (chl a) rich surface waters, but significant feeding also occurred in the deep layer. Gut pigment content throughout the water column varied from 0.8 to 22.0 ng pigment female−1 in April and from 0.2 to 21 ng pigment female−1 in September 1995. From the diel vertical migration pattern, it was estimated that female C. euxinus spend 7.5 h day−1 in April and 10.5 h day−1 in September in the chl a rich surface waters. Daily consumption by female C. euxinus in chl a rich surface waters was estimated by taking into account the feeding duration and gut pigment concentrations. Daily carbon rations of female C. euxinus, derived from herbivorous feeding in the euphotic zone, ranged from 6% to 11% of their body carbon weight in April and from 15% to 35% in September. Oxygen consumption rates of female and copepodite stage V (CV) C. euxinus were measured at different temperatures and at different oxygen concentrations. Oxygen consumption rates at oxygen-saturated concentration ranged from an average of 0.67 μg O2 mg−1 dry weight (DW) h−1 at 5°C to 2.1 μg O2 mg−1 DW h−1 at 23°C for females, and ranged from 0.48 μg O2 mg−1 DW h−1 at 5°C to 1.5 μg O2 mg−1 DW h−1 at 23°C for CVs. The rate of oxygen consumption at 16°C varied from 0.62 μg O2 mg−1 DW h−1 at 0.65 mg O2 l−1 to 1.57 μg O2 mg−1 DW h−1 at 4.35 mg O2 l−1 for CVs, and from 0.74 μg O2 mg−1 DW h−1 at 0.57 mg O2 l−1 to 2.24 μg O2 mg−1 DW h−1 at 4.37 mg O2 l−1 for females. From the oxygen consumption rates, daily requirements for the routine metabolism of females were estimated, and our results indicate that the herbivorous daily ration was sufficient to meet the routine metabolic requirements of female C. euxinus in April and September in the Black Sea.

Spatial distribution and trophic ecology of dominant copepods associated with turbidity maximum along the salinity gradient in a highly embayed estuarine system in Ariake Sea, Japan

The present study aimed to investigate into the feeding ecology of the dominant copepods along a salinity gradient in Chikugo estuary. Copepod composition was studied from samples collected from stations positioned along the salinity gradient of the estuary. Copepod gut pigment concentrations were measured by fluorescence technique and hydrographical parameters such as temperature, salinity, transparency, suspended particulate matter (SPM); pigments such as chlorophyll- a (Chl- a), phaeopigment; and particulate nutrients such as particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured. Two distinct zones in terms of nutrient and pigment concentrations as well as copepod distribution and feeding were identified along the estuary. We identified a zone of turbidity maximum (TM) in the low saline upper estuary which was characterized by having higher SPM, higher POC and PON but lower POC:PON ratios, higher pigment concentrations but lower Chl- a/SPM ratios and higher copepod dry biomass. Sinocalanus sinensis was the single dominant copepod in low saline upper estuary where significantly higher concentrations of nutrients and pigments were recorded and a multispecies copepod assemblage dominated by common coastal copepods such as Acartia omorii, Oithona davisae and Paracalanus parvus was observed in the lower estuary where nutrient and pigment concentrations were lower. Copepods in the estuary are predominantly herbivorous, feeding primarily on pigment bearing plants. However, completely contrasting trophic environments were found in the upper and the lower estuary. It was speculated from the Chl- a and phaeopigment values that copepods in the upper estuary receive energy from a detritus-based food web while in the lower estuary an algal-based food web supports copepod growth. Overall, the upper estuary was identified to provide a better trophic environment for copepod and is associated with higher SPM concentrations and elevated turbidity. The study demonstrates the role of estuarine turbidity maximum (ETM) in habitat trophic richness for copepod feeding. The study points out the role of detritus-based food web as energy source for the endemic copepod S. sinensis in the upper estuary, which supports as nursery for many fish species.

Errata

Environmental conditions and overwintering strategies of planktonic metazoans in and below coastal fast ice in the Gulf of Finland (Baltic Sea) Sarsia 89: 102–116 The following error appears in the abstract of the print version of the above paper published in issue 2, 2004 of the journal. The abstract is reproduced correctly below: A field study was conducted in Santala Bay with weekly samplings during February and March 2000. Ice thickness was 20–28 cm, snow cover 0–1 cm. The under‐ice water column was stratified with a cold (−0.3–0.2°C) and less saline (S = 2.1–4.9) interface layer. Concentrations of particulate organic carbon (0.5–5.8 mg POC l ) and algal pigments (0.3–18.2 μg chlorophyll a l ) were higher in the ice than in the water (0.2–0.5 mg POC l , 1.6–7.1 μg chlorophyll a l ) and peaked mostly in the bottom part of the ice. The thin ice and almost lacking snow cover had favoured an early ice‐algal and phytoplankton bloom. The diversity of metazoans was low, with six species in the ice and eight species in the under‐ice water. The rotifer Synchaeta cf. littoralis dominated both in ice and water, with maximum abundances of 230 individuals l in the bottom part of the ice. Rotifer eggs were also observed in the ice. Baltic sea ice seems to be a suitable habitat for rotifers. Nauplii and copepodids of the calanoid Acartia bifilosa in the under‐ice water showed some herbivorous feeding (<0.1–0.23 ng gut pigment individual ), but analysis of fatty acids, fatty alcohols and biomarker ratios indicated a more omnivorous/carnivorous diet. Despite low temperatures, this copepod showed growth and development below the ice, doubling in numbers (mainly CI, CII) from 118 to 230 individuals m during the third week of March.

Feeding and respiration rates of a planktonic copepod (Calanus sinicus) oversummering in Yellow Sea Cold Bottom Waters

The distribution, feeding and oxygen consumption of Calanus sinicus were studied in August 2001 on a transect across Yellow Sea Cold Bottom Waters (YSCBW) and two additional transects nearby. The distribution of C. sinicus adults and copepodites stage CV appeared to be well correlated with water temperature. They tended to concentrate in the YSCBW (>10,000 ind. m(-2)) to avoid high surface temperature. Gut pigment contents varied from 0.44 to 2.53 ng chlorophyll a equivalents (chl a equiv.) ind.(-1) for adults, and from 0.24 to 2.24 ng chl a equiv. ind.(-1) for CV copepodites. We found no relationship between gut pigment contents and the ambient chl a concentrations. Although the gut evacuation rate constants are consistent with those measured for other copepods, their low gut pigment contents meant an estimated daily herbivorous ingestion of <3% of body carbon in the YSCBW and <10% outside the YSCBW. However, based on estimates of clearance rates, C. sinicus feeds actively whether in the YSCBW or not, so the low ingestion rates probably reflect shortage of food. Oxygen consumption rates of C. sinicus ranged from 0.21 to 0.84 mul O-2 ind.(-1) h(-1), with high rates often associated with high temperature. From the oxygen consumption rates, daily loss of body carbon was estimated to be 4.0-13.7%, which exceeds our estimates of their carbon ingestion rates. C. sinicus was probably not in diapause, either within or outside the YSCBW, but this cold-water layer provides C. sinicus with a refuge to live through the hot, low-food summer.

Diel variations of copepod feeding and grazing impact in the high‐nutrient, low‐chlorophyll zone of the equatorial Pacific Ocean (0°; 3°S, 180°)

Diel variations of copepod biomass and feeding were studied at two time series stations in the high‐nutrient, low‐chlorophyll region of the central equatorial Pacific (0° and 3°S, 180°). During 48‐hour studies at each station, samples were taken at 3‐hour frequency in the neuston layer (0–1 m) and over the 0–100 m depth range. Feeding rates were assessed through the spectrofluorometric analysis of gut pigment contents. During the cycles, significant variations of biomass and gut pigment contents were found in both the neuston layer and the 0–100 depth strata. However, the amplitude of day‐night variations of feeding was larger and more clearly related to the day‐night cycle in the neuston layer than in the water column. Maxima in biomass and gut pigment contents did not coincide in either the surface or the water column. The differences in the mean copepod feeding activities at the equator and at 3°S were related to the mean standing stocks of chl a and to the pattern of undegraded pigments, which were different at the surface and in the water column, especially at the equator. In the surface layer, gut pheopigment minima were observed during the day, and maxima at night. This pattern clearly followed the periodicity of diel vertical migration, with intense feeding observed after the main upward ascent around sunset. Conversely, the feeding pattern in the integrated water column was more related to food abundance with a higher feeding during the day and around sunset, especially at the equator. Gut pigment contents in the water column displayed late afternoon maxima and dawn minima, coinciding with the maxima and minima of depth‐integrated chlorophyll a (chl a). On the basis of standing stocks of chl a containing particles larger than 3 μm, mean grazing pressure (i.e., specific grazing rates multiplied by copepod dry weights and divided by in situ chl a) varied between 3.0 and 4.0% in the upper 100 m and between 3.5 and 3.8% in the neuston layer. Because of the cyclical nature of feeding activities, short‐term variability in grazing pressure estimates were substantial, with individual estimates ranging from a high of 17.5% d−1 to a low of 0.2% d−1 in the neuston layer and from 0.9 to 7.8% d−1 in the 0–100 m water column.

Grazing impact of copepods on phytoplankton in the Bohai Sea

During spring (April/May 1999) and autumn (September/October 1998) cruises in the Bohai Sea, China, copepods were the dominant components of mesozooplankton, the most abundant species being Calanus sinicus, Centropages mcmurrichi, Paracalanus parvus, Acartia bifilosa and Oithona similis. Pigment ingestion rates by three size classes of copepods (200–500, 500–1000 and >1000 μm) were measured. In the south of the investigation area, gut pigment content (GPC), individual pigment-specific ingestion rates and grazing impacts on phytoplankton were lower in spring than in autumn. In the central area, GPC and individual pigment-specific ingestion rates were higher in spring than in autumn. The grazing impact on phytoplankton by the copepod assemblages was lower in spring than in autumn, however, because of the relatively smaller biomass in spring. In the western area where the Bohai Sea joins the Yellow Sea, GPC, individual pigment-specific ingestion rates and grazing impacts on phytoplankton were higher in spring than in autumn. Among the three size groups, the small-sized animals (200–500 μm) contributed more than 50% (range 38–98%) of the total copepod grazing during both cruises. The grazing impact on phytoplankton by copepods was equivalent to 11.9% (range 3.0–37.1%) of the chlorophyll- a standing stock and 53.3% (range 21.4–91.4%) of the primary production during the spring cruise. Grazing impact was equivalent to 6.3% (range 2.0–11.6%) of the chlorophyll- a standing stock and >100% (range 25.7–141.6%) of the primary production during the autumn cruise. The copepod community apparently consumed only a modest proportion of the standing stock of phytoplankton during spring and autumn blooms. They did, however, sometimes graze a significant proportion of daily primary production and hence were presumably able to limit the rate of further accumulation of phytoplankton, or even to prevent it.

Spring mesozooplankton communities in the epipelagic Ionian Sea in relation to the Eastern Mediterranean Transient

The present work on mesozooplankton of the Ionian Sea is part of a study aimed at assessing if the epipelagic communities had been affected by the changes in circulation (named EMT, Eastern Mediterranean Transient) observed in the eastern Mediterranean starting from 1988 as a transient effect of climate forcing. A temporal comparison was performed on data obtained in the springs of 1999 and 1992, periods characterized by opposite patterns in the upper circulation that in 1998 reversed from an anticyclonic to a cyclonic flow. A spatial comparison was performed on the data collected in 1999 from areas that previous oceanographic surveys had shown to be diversely affected by the EMT. Interannual differences were observed in the distribution of mesozooplankton abundance in the upper layer that might be related to the EMT dynamics and the effects of the reversed circulation. In 1999, abundance and composition of epipelagic mesozooplankton differed between the northwestern and eastern areas of the Ionian Sea. The distribution of species, trophic groups and copepod gut pigments suggested that different pelagic food webs took place at the opposite sides of the basin. A “classic” type food web prevailed in the northwestern, more productive area, whereas the microbial links prevailed in the eastern, more oligotrophic area. The characteristics of the northwestern area may in large part be due to the enrichment effects of the cyclonic circulation enhanced by the nutricline uprise due to the EMT, and to the interaction between the cyclonic circulation and the continental slope.

Feeding rhythm and vertical migration of the euphausiid Euphausia pacifica in coastal waters of north-eastern Japan during fall

We investigated the feeding rhythm and diel vertical migration of Euphausia pacifica in coastal waters of north-eastern Japan in September 2000. The euphausiids arrived at the surface early in the night and sank just after satiation. Gut pigment contents and stomach fullness were higher in the food-rich surface layer at night than in the food-poor deep layer during the day. Feeding activity of E. pacifica during daytime was low even if food organisms were abundant in the ambient water in the top 50 m. Therefore, the feeding rhythm of E. pacifica is explained by decreased feeding activity during the day rather than by reduced food availability.

Mesozooplankton community structure and grazing impact in the Polar Frontal Zone of the south Indian Ocean during austral autumn 2002

Mesozooplankton community structure and grazing impact were investigated at 13 stations in the Polar Frontal Zone during the second Marion Offshore Variability Ecosystem Study (MOVES II), conducted during April 2002. Total integrated chl- a biomass ranged between 11.17 and 28.34 mg chl- a m-2 and was always dominated by nano- and picophytoplankton (<20 µm). Throughout the study, small copepods, mainly Oithona similis and Ctenocalanus vanus, numerically dominated the mesozooplankton community, composing up to 85% (range 30–85%) of the total abundance. Grazing activity of the four most abundant copepods ( O. similis, C. vanus, Calanus simillimus and Clausocalanus spp.), constituting up to 93% of total mesozooplankton abundance, was investigated using the gut fluorescence technique. Results of gut fluorescence analyses indicated that Calanus simillimus and Clausocalanus spp. exhibited diel variability in gut pigments with maximum values recorded at night. In contrast, O. similis and Ctenocalanus vanus did not demonstrate diel variation in gut pigment contents. Ingestion rates of the four copepods ranged from 23.23 to 1462,02 ng(pigm.)ind-1day-1. The combined grazing impact of the four copepods ranged between 1 and 36% of the phytoplankton standing stock per day, with the highest daily impact occurring at stations occupied in the vicinity of the Antarctic Polar Front (~35.86% at station 23). Among the copepods, O. similis and Ctenocalanus vanus represented the most important consumers of phytoplankton biomass, collectively responsible for up to 89% (range 15–89%) of the total daily grazing impact. Carbon specific ingestion rates of the copepods varied between 42 and 320% body carbon per day.

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual–1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual–1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual–1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual–1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h–1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h–1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m–2 day–1 and accounted for 34–69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m–2 day–1, accounting for 17–41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.