Zooplankton feeding and queueing theory

Three of the most commonly used mathematical relationships between zooplankton ingestion and food concentration are derived from a queue model, by assuming different feeding strategies of the individual predators or grazers. The model suggests that the abruptness of the transition between maximum search or filtration rate and maximum ingestion rate depends on the size ratio between the animals and their food items. The results show that the stochastic nature of predation and grazing cannot be neglected when the food particles are relatively big.     

The use of measurable coefficients in process formulations—zooplankton grazing

This paper redefines a construct previously used to model phytoplankton—zooplankton interactions in such a way as to permit the use of measurable quantities as contruct coefficients. The new construct can use unaltered values of the half-saturation constant for zooplankton grazing on total available food (k_s) and the minimum food concentration necessary to stimulate effective feeding (BMIN) reported in the literature. Typical values for these coefficients are 0.1–15 and 0.016–0.19, respectively.     

Microzooplankton grazing and selectivity of phytoplankton in coastal waters

Microzooplankton grazing activity in the Celtic Sea and Carmarthen Bay in summer 1983 and autumn 1984 was investigated by applying a dilution technique to high-performance liquid chromatographic (HPLC) analysis of photosynthetic pigments in phytoplankton present within natural microplankton communities. Specific grazing rates on phytoplankton, as measured by the utilisation of chlorophyll a, were high and varied seasonally. In surface waters during the autumn, grazing varied between 0.4 d^-1 in the bay and 1.0 d^-1 in the Celtic Sea, indicating that 30 and 65% of the algal standing stocks, respectively, were grazed daily. Grazing rates by microzooplankton within the thermocline in summer suggest that 13 to 42% of the crop was grazed each day. Microzooplankton showed selection for algae containing chlorophyll b, in spite of a predominance of chlorophyll c within the phytoplankton community. Changes in taxon-specific carotenoids indicated strong selection for peridinin, lutein and alloxanthin and selection against fucoxanthin and diadinoxanthin. This indicates a trophic preference by microzooplankton for dinoflagellates, cryptophytes, chlorophytes and prasinophytes and selection against diatoms, even when the latter group forms the largest crop within the phytoplankton. Interestingly, those algal taxa preferentially grazed also showed the highest specific growth-rates, suggesting a dynamic feed-back between microzooplankton and phytoplankton. Conversion of grazing rates on each pigment into chlorophyll a equivalents suggests firstly, that in only one experiment could all the grazed chlorophyll a be accounted for by the attrition of other chlorophylls and carotenoids, and secondly that in spite of negative selection, a greater mass of diatoms could be grazed by microzooplankton than any other algal taxon. The former may be due either to a fundamental difference in the break-down rates of chlorophyll a compared to other pigments, or to cyanobacteria forming a significant food source for microzooplankton. In either case, chlorophyll a is considered to be a good measure of grazing activity by microzooplankton.     

Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Trophic status and feeding selectivity of blennies (Blenniidae: Salariini)

An evaluation of the dietary resources available to, and selected by, blennies of the tribe Salariini was used to assess their trophic status on the Great Barrier Reef. Gut-content analysis of nine species of blennies found detrital aggregates to be the dominant item ingested. Samples of the dietary resources available to one of these species, Salarias patzneri, were compared biochemically. Of the two main dietary categories (detrital aggregates and filamentous algae), detrital aggregates were the major resource available, accounting for 53 ± 4.6% (SE) of the organic matter present. The mean C:N values for detrital aggregates (17.2 ± 0.8) and filamentous algae (20.0 ± 1.8) were very similar, as were protein concentrations (1.8 ± 0.1 and 2.1 ± 0.1 μg mg⁻¹, respectively). However, mean carbohydrate concentrations were much lower in the aggregates than the filamentous algae (19 ± 2 and 76 ± 13 μg mg⁻¹, respectively). Comparison of the inorganic particles ingested to those available indicated that S. patzneri selectively fed on particles <125 μm, (predominantly detrital aggregates) and avoided particles >250 μm, (predominantly algal filaments). The patterns of resource availability, nutritional quality and selectivity suggest that salariin blennies utilise detrital aggregates as their primary dietary resource. Received: 24 May 1999 / Accepted: 29 November 1999     

Zooplankton grazing on the coccolithophore Emiliania huxleyi and its role in inorganic carbon flux

Grazing and faecal pellet production by the copepods Calanus helgolandicus and Pseudocalanus elongatus, feeding on the coccolithophore Emiliania huxleyi, were measured under defined laboratory conditions, together with the chemical characteristics and sinking rates of the faecal pellets produced. Ingestion rates of both copepods were equivalent at comparable cell concentrations, the relationship between ingestion rate (I, cells copepod^-1 h^-1) and food concentration (C, cells ml^-1), being I=0.558C for both species. P. elongatus produced a larger number of smaller faecal pellets than C. helgolandicus, but egested a larger volume of material per individual. Only between 27 and 50% of the ingested coccolith calcite was egested in the faecal pellets, and it is possible that acid digestion in the copepod gut is responsible for these considerable losses. Average sinking rates of faecal pellets containing E. huxleyi coccoliths, produced by both species, were >100 m d^-1. The implications of the quantitative laboratory estimates for the vertical flux of inorganic carbon are considered using recently studied shelf-break and oceanic E. huxleyi blooms in the N. E. Atlantic as examples.     

Zooplankton biomass and indices of grazing and metabolism during a late winter bloom in subtropical waters

The development of the so-called late winter bloom in subtropical water was studied in an oceanic area north of the Canary Islands from January to May 2000. Zooplankton was sampled at short-term intervals (1–4 days) during the bloom (January–March), and biomass, indices of grazing (gut fluorescence) and metabolism (electron transfer system activity, ETS) were measured in four different size fractions (100–200, 200–500, 500–1000 and >1000 µm). During the bloom, ETS activity and gut fluorescence increased before the development of zooplankton biomass. At the end of February, the presence of an impressive cloud of dust formed in the Sahara desert was related to an increase in chlorophyll and small zooplankton a week later. The increments in biomass were the consequence of consumption by zooplankton as inferred from the indices of grazing and metabolism. Estimated grazing from gut fluorescence and gut evacuation rates during the period of study accounted for 55% of the assessed total ingestion from respiration and normal values of assimilation, showing the importance of the non-pigmented food in the diet of zooplankton in these waters. In contrast, the sharp decreases in zooplankton biomass observed during the bloom appeared during the dark period of the moon, the days in which the diel vertical migrants reach the shallower layers, in agreement with previous works in the area. Thus, the development of the late winter bloom in this region is suggested to be driven by the interplay between resource and consumer controls.Communicated by S.A. Poulet, Roscoff     

Patterns and selectivity in the feeding of certain mesopelagic fishes

Stomach contents were analysed from the 7 most numerous species of mesopelagic fish caught in a series of 11 hauls over a 24 h period at 230 to 266 m depth in the eastern North Atlantic Ocean. The numerical abundance of organisms per filled stomach and the frequency of occurrence of empty stomachs were used to indicate feeding periodicity. The ecological significance of the feeding periodicity was considered by examining it in connection with an investigation of the day-night vertical distribution of zooplankton and micronekton to 2000 m at the same station. Additional dietary evidence on the 7 species considered was also obtained from the vertical series. Feeding selectivity was examined by comparing the composition of the zooplankton population, sampled separately but simultaneously with the micronekton, with that from the overall stomach contents of the species examined. Feeding periodicity was demonstrated for 6 species, of which 3 were found to be feeding selectively: Valenciennellus tripunctulatus on calanoid copepods, Argyropelecus aculeatus on ostracods, and Lampanyctus cuprarius on amphipods and possibly euphausiids. The limited data available on the other 3 species suggested that they were either random feeders (A. hemigymnus and Lobianchia dofleini) or perhaps selecting against a particular group (Notolychnus valdiviae). No indication of feeding periodicity or selectivity was found for Chauliodus danae. The overall pattern of results confirmed the supposed close correlation between vertical migration and feeding in mesopelagic fish.     

Food selectivity of the rotifer Brachionus plicatilis feeding on phytoplankton

Food selectivity was examined in amictic female rotifers, Brachionus plicatilis, fed Chlamydomonas sp. and Olisthodiscus sp. Filtering and ingestion rates of the rotifers in a single food suspension were higher with Chlamydomonas sp. than with Olisthodiscus sp. In every mixture of two food species, the apparent filtering rate on Chlamydomonas sp. was much higher than that on Olisthodiscus sp. Total filtering rate and apparent filtering rate in food suspensions containing Chlamydomonas sp. decreased with increasing cell concentration of Chlamydomonas sp. Total and apparent filtering rates were calculated from the decrease in total food concentration as well as from decreases in concentrations of each of the two food algae, respectively. B. plicatilis ingested Olisthodiscus sp. at an extremely low constant rate in all mixtures. The degree of food selectivity of the rotifers fed Chlamydomonas sp. (i.e., selective filtration) from the mixture of two food algae decreased with increasing cell concentration of Chlamydomonas sp. Filtering and ingestion rates of rotifers fed senescent Chlamydomonas sp. were relatively lower than those fed Chlamydomonas sp. in the exponential phase. This indicates that Brachionus plicatilis displays selectivity in regard to condition of cells as well as type of food.     

Growth and grazing responses of tintinnid ciliates feeding on the toxic dinoflagellate Heterocapsacircularisquama

Growth and feeding rates of two tintinnid species, Favellaazorica and Favellataraikaensis, were determined under various concentrations of the dinoflagellate Heterocapsacircularisquama which has been reported as highly toxic to shellfish. Mean growth rates of F. azorica and F. taraikaensis on a diet of H.circularisquama (ca. 10² cells ml⁻¹) were 2.15 and 1.97 doublings d⁻¹, respectively. These values are similar to those on a diet of Heterocapsatriquetra which is suitable food for various zooplankton. However, growth rates of both tintinnid species decrease with increasing concentrations of >10³ cells ml⁻¹ of H. circularisquama. In particular, H. circularisquama under conditions of >10³ cells ml⁻¹ caused mortality in F.taraikaensis, probably due to toxins. Clearance and ingestion rates of F. azorica on H. circularisquama were 4.1 to 27.5 μl ind⁻¹ h⁻¹ and 1.5 to 28.7 cells ind⁻¹ h⁻¹, respectively, at concentrations of <10⁴ cells ml⁻¹ and those of F. taraikaensis were 0.9 to 22.1 μl ind⁻¹ h⁻¹ and 0.1 to 13.0 cells ind⁻¹ h⁻¹, respectively, at concentrations of <10³ cells ml⁻¹. Both clearance and ingestion rates on H.circularisquama were higher for replicates fed on H.triquetra. Daily grazing impact of the two species of Favella on the initial stage of a bloom of H.circularisquama were estimated to reach 6 to 50% of H. circularisquama at a concentration of 540 cells ml⁻¹, indicating that grazing by tintinnids such as Favella spp. may significantly regulate the initial stages of blooms of H. circularisquama. Received: 3 January 1997 / Accepted: 27 January 1997     

Filter feeding by Oikopleura vanhoeffeni: grazing impact on suspended particles in cold ocean waters

Because of the abundance and size of Oikopleura vanhoeffeni its quantitative role as a suspension feeder in cold ocean waters needs to be defined. To minimize the effect of manipulation and containment, and to assess the effect of naturally occurring factors on clearance rate, I used an in situ latex microbead technique in Logy Bay, Newfoundland, from February 1985 to June 1986. Individual clearance rates ranged from 8–944 ml h^-1, increasing exponentially with increasing trunk length. Partial correlation and principal components analysis indicated that trunk length and the concentration of ingestible chlorophyll a accounted for a majority of the variation in clearance rate. At densities of 4–110 m^-3, O. vanhoeffeni populations removed from >1 to 13% of the standing stock of ingestible food particles each day. Grazing by near-surface populations was lowest during the spring diatom bloom (>1.4% of daily particle production removed per day), and was highest in June during the post-bloom crash (4 to 10% of daily production removed). Some populations in mid-depth waters had much higher population clearance rates (ca. 50% of daily production removed) because of a greater proportion of large animals. The median percentage daily ration (g Cxg C^-1xd^-1x100%) of 64% accounted for observed house production rates (1 to 2 d^-1, with each house=23% of body carbon).     

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.     

Zooplankton fisheries of the world: A review

About 20 species of zooplankters (copepods, mysids, euphausiids, sergestids, and Scyphomedusae) are commercially fished and utilised as food or feed today. The annual world catch of crustacean zooplankton is probably a little less than 210,000 tons and accounts for 11% of the total crustacean catch in the world. The present status of plankton harvesting in various parts of the world is described. Problems in development of plankton fisheries are discussed.     

Description and use of an improved method for determining estuarine zooplankton grazing rates on phytoplankton

A method of rapidly determining zooplankton grazing rates on natural mixed phytoplankton populations using ¹⁴C is described. The method simplifies the design of grazing experiments as the grazing time can be kept short enough to prevent recycling of the isotope, and growth of the phytoplankton substrate. Very high specific activity, ¹⁴C-labelled phytoplankton concentrated either by centrifugation or sieving, may be used either as the sole grazing substrate, or as a tracer in natural mixed phytoplankton. Zooplankton, confined in glass jars at either ambient, or higher than ambient concentrations, are permitted to feed on the phytoplankton for periods of 30 min and 2 h, and are then separated by sieving. The zooplankton community grazing rate, or, if the samples are sorted into species, the individual species grazing rates, can be determined after scintillation counting of the zooplankton. The rate of appearance of ¹⁴C-labelled phytoplankton in the zooplankton is an estimate of the grazing rate, and the slope of the line joining the grazing rates at various phytoplankton concentrations gives an estimate of the grazing rate constant for the zooplankton population. The method provides a quick way of obtaining both zooplankton population, and individual species grazing rates on natural mixed phytoplankton. In two experiments, labelled phytoplankton was used as the sole grazing substrate in concentrations ranging between 0.4 and 5 times ambient levels. Grazing rate constants, for net-caught zooplankton concentrated to 46 times (Experiment 1) and 28 times (Experiment, 2) ambient estuarine levels were-0.14and-0.12 of the phytoplankton standing stock per day, respectively. There was a linear increase in the amount of phytoplankton grazed with an increase in phytoplankton concentration up to four times ambient phytoplankton levels. When tracer amounts of labelled phytoplankton were added to samples containing both phytoplankton and zooplankton at ambient concentrations the grazing rate constants were-0.28 and-0.42 of the phytoplankton standing stock per day. We conclude that zooplankton grazing was the major control factor of phytoplankton population size during October–November 1975 in South West Arm, Port Hacking, near Sydney, Australia.     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

Zooplankton biodiversity and lake trophic state: explanations invoking resource abundance and distribution

While empirical studies linking biodiversity to local environmental gradients have emphasized the importance of lake trophic status (related to primary productivity), theoretical studies have implicated resource spatial heterogeneity and resource relative ratios as mechanisms behind these biodiversity patterns. To test the feasibility of these mechanisms in natural aquatic systems, the biodiversity of crustacean zooplankton communities along gradients of total phosphorus (TP) as well as the vertical heterogeneity and relative abundance of their phytoplankton resources were assessed in 18 lakes in Quebec, Canada. Zooplankton community richness was regressed against TP, the spatial distribution of phytoplankton spectral groups, and the relative biomass of spectral groups. Since species richness does not adequately capture ecological function and life history of different taxa, features which are important for mechanistic theories, relationships between zooplankton functional diversity (FD) and resource conditions were examined. Zooplankton species richness showed the previously established tendency to a unimodal relationship with TP, but functional diversity declined linearly over the same gradient. Changes in zooplankton functional diversity could be attributed to changes in both the spatial distribution and type of phytoplankton resource. In the studied lakes, spatial heterogeneity of phytoplankton groups declined with TP, even while biomass of all groups increased. Zooplankton functional diversity was positively related to increased heterogeneity in cyanobacteria spatial distribution. However, a smaller amount of variation in functional diversity was also positively related to the ratio of biomass in diatoms/chrysophytes to cyanobacteria. In all observed relationships, a greater variation of functional diversity than species richness measures was explained by measured factors, suggesting that functional measures of zooplankton communities will benefit ecological research attempting to identify mechanisms behind environmental gradients affecting diversity.     

Zooplankton growth rates: the influence of size and resources in tropical marine copepodites

Growth rates were determined for copepodites of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total copepodite development time was as short as 4 to 5 d. Mean instantaneous growth rates (g) ranged from as high as 1.2 d⁻¹ to as low as 0.1 d⁻¹. In general, cyclopoid copepods appeared to grow more slowly than calanoids of the same size. Enhancement of resources by nutrient addition caused a 32% increase in growth rates in experiments from a mesotrophic site, but only a 17% increase at a more eutrophic site. Additionally, copepodites at both sites showed faster development and generally larger size at stage in response to nutrient addition. Growth rates were positively related to chlorophyll concentration in the >2 μm size-fraction. A significant relationship of growth rate to body size (r ² = 0.45) emerged across a wide range of trophic status, but it was confounded with resource availability. It appears that growth in tropical copepod copepodites may be frequently limited by resources in a size-dependent manner. Received: 30 May 1997 / Accepted: 13 May 1998     

Zooplankton responses to hypoxia: behavioral patterns and survival of three species of calanoid copepods

Seasonally recurrent and persistent hypoxic events in semi-enclosed coastal waters are characterized by bottom-water dissolved oxygen (d.o.) concentrations of < 2.0 ml l⁻¹. Shifts in the distribution patterns of zooplankters in association with these events have been documented, but the mechanisms responsible for these shifts have not been investigated. This study assessed interspecific differences in responses to hypoxia by several species of calanoid copepods common off Turkey Point, Florida, USA: Labidocera aestiva (Wheeler) (a summer/fall species), Acartia tonsa (Dana) (a ubiquitous year-round species), and Centropages hamatus (Lilljeborg) (a winter/spring species). Under conditions of moderate to severe hypoxia 24-h survival experiments were conducted for adults and nauplii of these species from August 1994 to October 1995. Experiments on adults used a flow-through system to maintain constant d.o. concentrations. Adults of A. tonsa showed no decline in survival with d.o. as low as 1.0 ml l⁻¹, sharp declines in survival at d.o. = 0.9 to 0.6 ml l⁻¹, and 100% mortality with d.o. = 0.5 ml l⁻¹. Adults of L. aestiva and C. hamatus were more sensitive to oxygen depletion: both species experienced significant decreases in survival for d.o. = 1.0 ml l⁻¹. Nauplii of L. aestiva and A. tonsa showed no significant mortality with d.o. = 1.1 to 1.5 ml␣l⁻¹ and d.o. = 0.24 to 0.5 ml l⁻¹, respectively. In addition, experiments investigating behavioral avoidance of moderate to severe hypoxia were carried out for adults of all three species. None of the three species effectively avoided either severely hypoxic (d.o. < 0.5 ml l⁻¹) or moderately hypoxic (d.o. ≈ 1.0 ml l⁻¹) bottom layers in stratified columns. These results suggest that in␣nearshore areas where development of zones of d.o. < 1.0 ml l⁻¹ may be sudden, widespread, or unpredictable, patterns of reduced copepod abundance in bottom waters may be due primarily to mortality rather than avoidance. Received: 31 August 1996 / Accepted: 24 September 1996     

Zooplankton growth rates: the influence of size in nauplii of tropical marine copepods

Growth rates and development times were determined for nauplii of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total naupliar development time was short: 3 to 4 d inshore and 4 to 5 d offshore. Mean instantaneous growth rates (g) ranged from as high as 0.90 d⁻¹ for Parvocalanuscrassirostris to as low as 0.41 d⁻¹ for Corycaeus spp. In general, nauplii of cyclopoid copepods appeared to grow more slowly than those of calanoids of the same size. Naupliar growth rates were significantly related to body size (r ²= 0.43 to 0.50), but were unrelated to chlorophyll concentration in any measured size-fraction. This suggests that nauplii are generally not limited by resources, but are growing at their maximum temperature and size-dependent rates. Received: 30 May 1997 / Accepted: 13 May 1998     

Zooplankton: pre-southwest and northeast monsoons of 1993 to 1994, from the North Arabian Sea

Zooplankton samples from the North Arabian Sea Environment and Ecosystem Research (NASEER) cruises were analyzed to determine the basic taxonomic composition, biomass (standing stock) and the total and copepod numeric abundance; these characteristics are discussed with reference to the different monsoon periods. Cruises carried out during March 1993 and May 1994, categorized as pre-southwest monsoon periods, and a cruise in December 1994, categorized as a northeast monsoon period, are discussed in detail. The biomass of January 1992 versus August 1992 and August 1992 versus March 1993 differed significantly (F = 6.44, P≤ 0.05). Ranges of highest and lowest biomass from each cruise are also given. Distinct “high” and “low” production areas of statistically significant difference (F = 12.67, P≤ 0.05) were observed. The “high” and “low” production areas were mobile and followed the surface wind circulation patterns (wind reversal pattern) during the northeast and southwest monsoon periods. Overall zooplankton showed a patchy distribution. The overall zooplankton abundance and total copepod counts differed significantly between the Cruises 3 versus 4 and 4 versus 5 (F = 15.67, P≤ 0.05 and F = 34.39, P≤ 0.05, respectively). There was no significant difference (P≥ 0.05) in biomass, between eutrophic and oligotrophic stations, suggesting no difference between near shore and offshore waters. Thirty-eight taxonomic groups were identified from the samples, with copepods being the most dominant group, followed by chaetognaths and siphonophores. Copepods constitute an average of 52.50 to 74.93% of the total zooplankton count and reach maxima of 92.14% of the total zooplankton count at the outset of the southwest monsoon (March 1993) and 91.39% at the outset of the active northeast monsoon (December 1994). Received: 27 February 1998 / Accepted: 8 October 1999