Nitrogen excretion by some demersal macrozooplankton in Heron and One Tree Reefs,Great Barrier Reef,Australia

Nitrogen excretion rates of demersal macrozooplankton were measured together with nitrogen concentrations in the water column and sediments in lagoons of Heron Reef and One Tree Reef, Great Barrier Reef, Australia, during August and November 1991. Excretion rates increased with body weight, and weight-specific excretion rates of the demersal macrozooplankton were comparable to those of pelagic zooplankton and meiofauna in the Great Barrier Reef. Values of demersal macrozooplankton abundance from previous studies and excretion rates from this study were combined to estimate fluxes of ammonium from demersal macrozooplankton in coral reef lagoons. The estimated fluxes in the water column and sediments were 12 M NH₄ m^-2 d^-1 and 34 M NH₄ m^-2d^-1, respectively. These fluxes were compared with reported fluxes of ammonium in coral reef lagoons in the Great Barrier Reef, Australia. The estimated flux from the demersal macrozooplankton in the water column was 29 and 9% of those reported for microheterotroph regeneration and phytoplankton utilization, respectively. It was 10% of the reported advective flux during periods of low advection and 13% of the maximum efflux from sediments computed from diffusion models. The estimated flux from the demersal macrozooplankton in the sediments exceeded those reported for meiofauna, and was 5 to 32% and 2 to 13% of those reported for ammonification and utilization in sediments, respectively. The potential importance of demersal macrozooplankton in mediating sediment-water column exchanges in the absence of diffusive effluxes and when they swarm is discussed.     

Zooplankton of tikehau atoll (Tuamotu archipelago) and its relationship to particulate matter

The standing stock and taxonomic composition of zooplankton (>200 m) were monitored in the lagoon of Tikehau atoll from April 1985 to April 1986. These data were supplemented by two 10 d studies on the variability, structure and functioning of the pelagic ecosystem. The biomass of animals >200 m comprised 50% of the total biomass of all organisms from 35 to >2000 m. The zooplankton populations were characterized by successive blooms of copepods, larvaceans, pteropods and salps, probably arising from the periodic input of detritus from the reef during windy periods. As a result, the ecosystem was not in a steady state and the data for the fluxes of organic matter are presented separately for April 1985 and April 1986. Using the C:N:P ratio method, net growth efficiencies, K ₂, were calculated for total mesozooplankton, mixed copepods, and two planktonic species, Undinula vulgaris and Thalia democratica. Combined with nitrogen and phosphorus excretion rates, these K ₂ values enabled the assessment of production rates. On a 24 h basis, P:B ratios (%) were close to 100 for the total zooplankton and 54, 34 and 800 for mixed copepods, U. vulgaris and T. democratica, respectively. These ratios are 5.7 times lower than that recorded for phytoplankton. High productivity may be ascribable to the high density of seston, the high temperature (29.5°C), and the kind of organisms present. Zooplankton production equalled 38 and 30% of ¹⁴C uptake during April 1985 and April 1986, respectively. Ingestion of animals >35m was calculated by means of assimilation efficiencies and amounted to 17 and 7% of particulate organic carbon, 100 and 38% of living carbon, and 64 and 140% of primary production during the two periods, respectively. Finally, inorganic exduring was 32 and 18% of phytoplankton nitrogen and phosphorus requirements. A model based on the dimensional structure of the pelagic food-web, has been drawn to illustrate the biomass and carbon, nitrogen and phosphorus fluxes in the study area. The lagoon appears to export part of its planktonic biomass, which is 4.2 times lower one sea mile outside the main pass connecting the lagoon to the open ocean.     

Distribution,abundance, and substrate preferences of demersal reef zooplankton at Lizard Island Lagoon,Great Barrier Reef

Demersal zooplankton, those plankton which hide within reef sediments during the day but emerge to swim freely over the reef at night, were sampled quantitatively using emergence traps planced over the substrate at Lizard Island Lagoon, Great Barrier Reef. Densities of zooplankton emerging at night from 6 substrate types (fine, medium, and coarse sand, rubble, living coral and reef rock) and from 5 reef zones (seaward face, reef flat, lagoon, back reef, and sand flat) were determined. A large population of nocturnal plankton including cumaceans, mysids, ostracods, shrimp, isopods, amphipods, crustacean larvae, polychaetes, foraminiferans and copepods are resident members of the reef community at Lizard Island. The mean density of plankton emerging throughout the reef was 2510±388 (standard error) zooplankton/m² of substrate. Biomass averaged 66.2±5.4 mg ash-free dry weight/m² of substrate. Demersal zooplankton exhibited significant preferences for substrate types and reef zones. The highest mean density of zooplankton emerged from coral (11,264±1952 zooplankton/m²) while the lowest emerged from reef rock (840±106 zooplankton/m²). The density of demersal plankton was six times greater on the face than in any other zone, averaging 7900±1501 zooplankton/m². Copepods dominated samples collected over living coral and rubble while foraminiferans, ostracods and decapod larvae were most abundant from sand. Plankton collected with nets at night correlated only qualitatively with plankton collected in emergence traps from the same location. Although abundant, demersal plankton were not numerous enough to meet the metabolic needs of all corals at Lizard Island Lagoon. Demersal plankton appear especially adapted to avoid fish predation. The predator-avoidance strategies of demersal plankton and maintenance of position on the reef are discussed. Our results indicate that much of the zooplankton over coral reefs actually lives on the reef itself and that previous studies using standard net sampling techniques have greatly underestimated plankton abundance over coral reefs.     

Marine food-web analysis: An experimental study of demersal zooplankton using isotopically labelled prey species

Short-term incubations in seawater containing H¹⁴CO₃ ^- or ³H₂O in place of the naturally predominant isotopes can yield highly radioactive preparations of living phytoplankton or zooplankton. Subsequent in situ incubation of these labelled organisms with the community from which they were taken results in the rapid transfer of radioisotope to those species which prey upon them. This technique has been employed to map a portion of a marine food web involving demersal zooplankton; experiments were conducted in summer and autumn on a coral reef and in a subtropical estuary. Similar results were obtained from these initial experiments at each study site during both seasons. Prey supplied as zooplankton (124 to 410 m nominal diameter), which consisted mainly of Oithona oculata, was fed upon by zooplankton size classes ranging from 410 to 850 m and containing amphipods, ostracods, cumaceans and polychaetes. In experiments employing labelled phytoplankton as prey a wide size spectrum was used (10 to 106 m) in order to include representative samples of most of the available planktonic autotrophs as estimated by primary production measurements. In two separate experiments, only 7 out of 63 samples evidenced grazing of phytoplankton by demersal zooplankters. In contrast, labelled diatom auxospores, employed in one experiment as they constituted the most numerically abundant species in the water column, were found to be grazed upon in nearly half the samples examined.     

Detrital pathways in a coral reef lagoon

Coral reef lagoons have generally been regarded as sinks for organic matter exported from more productive reef front and reef flat zones. The object of this study was to examine the importance of detritus as a carbon source for benthic communities in the lagoon at Davies Reef, central Great Barrier Reef. We report the results of seasonal measurements, taken in 1986, of bacterial numbers and production, protozoan numbers, community primary production and respiration in the sediments of Davies Reef lagoon. Deposition rates of organic matter in the lagoon were also measured. Deposition rates (±1 SE) of carbon ranged from 9.2 (±1.5) to 140.7 (±10.3) mg Cm^-2d^-1. Deposition rates were highest in winter and spring, lowest in summer. Rates of bacterial production ranged from 4.7 (±0.2) pmol thymidine incorporated g^-1 dry wt (DW) h^-1 in winter to 23.5 (±1.0) pmol thymidine incorporated g^-1 DW h^-1 in spring. The number of ciliates ranged from 65 (±10) to 356 (±50) cm^-3 through the year and the number of large (20 m) flagellates from 38 (±7) to 108 (±16) cm^-3. There were no clear relationships between the sediment organic content, detrital input or temperature and the rates of bacterial processes, community metabolism or the standing stocks of microbes in the lagoon. The relative significance of detritus and in situ primary production as sources of carbon in the lagoon varied with season. In summer and autumn, detritus was less important than primary production as a source of carbon (4 to 27% of total carbon input). In winter and spring, detritus input became more significant in supply of carbon to the sediments (32 to 67% of the total carbon input). The lagoon does not simply act as a sink for carbon exported from the reef flat. We calculate that only 5% of the net reef flat primary production reached lagoon sediments in summer, but nearly 40% in winter.     

Zooplankton community structure and water flow in the lee of Helix Reef (Great Barrier Reef, Australia)

To investigate which physical processes contribute most in moulding zooplankton community structure in the waters close to coral reefs, light traps moored in a grid pattern were used to collect zooplankton from the sea surface at 16 stations on the downstream side of Helix Reef during three time periods (2100-2200, 2400-0100, and 0300-0400 hours) over three consecutive nights covering the new moon period in January 1992. Two distinct zooplankton communities were present: a community composed primarily of reef-resident, demersal plankton immediately to the south of the reef in an area of reduced flushing, and a community containing coastal and shelf-seas taxa at the more exposed sites in the open flow field. The fauna composition at a number of exposed stations was as rich as that at sheltered stations both in terms of number of taxa and diversity indices but was almost an order of magnitude less abundant. The reef-resident, demersal plankton community was dominated by gammarid amphipods, mysids, and polychaetes, whereas only transient, meroplanktonic forms such as echinoderm and echinopluteus larvae and shelf-seas, holoplanktonic forms such as doliolids and larvaceans were significantly more abundant in the exposed community. Zooplankton associations were apparently formed by a combination of hydrodynamic processes, spatial and temporal distribution patterns of individual taxa, specific behaviours of certain taxa, and the interactions among taxa at different trophic levels.     

Spatial,temporal, and behavioral patterns in emergence of zooplankton in the lagoon of Heron Reef,Great Barrier Reef,Australia

A total of 34 zooplanktonic taxa were common in emergence trap, reentry trap, and net-tow samples taken in the lagoon of Heron Reef, Great Barrier Reef, between 27 February and 22 March and between 11 June and 4 July 1985. Twenty-nine of these taxa were classified as demersal (17 taxa), meroplanktonic and larval (10 taxa), or incidental (2 taxa). The remaining five multispecific groups yielded variable results. Differences were observed between two locations separated by 200 m, with emergence 2 to 62 times greater for 22 taxa at a deeper site with larger coral formations. Most zooplankters were more common (2 to 122 times) in samples from 1 m² areas around 0.25 to 0.5 m² patches of branching coral. However, two species of copepods, Pseudodiaptomus colefaxi and Metis holothuriae, were taken in greater numbers (3 to 12 times) from open sand. Seasonal increases (2 to 322 times) were observed for 13 adult taxa and 8 classes of larvae or juveniles in the summer and for 10 adult taxa and one larval group in the winter. Fourteen of the 17 demersal taxa and 4 groups of large larvae or juveniles emerged in numbers 2 to 323 times greater during lunar quarters or new moons. In contrast, 6 larval taxa and 4 groups of small or transparent adults displayed significant emergence during full moons. Five diurnal emergence patterns were shown by 27 taxa, with patterns varying primarily among lunar periods. The variety of taxon-specific patterns observed in this study highlight a need for caution when generalizing about demersal zooplankton.     

Lake of flies, or lake of fish? A trophic model of Lake Malawi

Ecosystem-focused models have, for the first time, become available for the combined demersal and pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to explore continuing controversies over the production efficiencies and ecological functioning of large tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries production. A mass-balanced trophic model developed for the demersal fish community of the southern and western areas of Lake Malawi was integrated with an existing trophic model developed for the open-water pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions, fish food consumption studies, and from additional published quantitative and qualitative information on the various biotic components of the community. The model was constructed using the Ecopath approach and software. The graphically presented demersal food web spanned four trophic levels and was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton was imported into the system at trophic levels three and four through fish predation on carnivorous and herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This evidence for strong benthic-pelagic coupling provided the opportunity for linking the demersal model to the existing model for the pelagic community so producing the first model for the complete ecosystem. Energy fluxes through the resulting combined model demonstrated that the primary import of biomass to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only 15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production is efficiently utilised by the lake fish community, and any attempt to improve fishery production through introduction of a non-native plantivorous fish species would have a negative impact on the stability and productivity of the lake ecosystem.     

Plankton inhabiting the surface layer of the southern and southwestern lagoon of New Caledonia

In the southern and southwestern coral reef lagoon of New Caledonia and the adjacent oceanic waters, 42 neuston samples were collected from the upper surface layer (0 to 10 cm) along nine transects from the coast to the coral barrier reef and the open ocean immediately beyond the reef, in March and April 1979. There was a progressive numerical decrease in zooplankton densities from the coast to the reef and from the reef to the open sea. Generally, 80 to 95% of the surface plankton consisted of holoplankton and 5 to 20% of meroplankton. Zooplankton was very abundant in littoral bays with a marked eutrophication. In a few samples collected in very shallow waters close to coral patches, cladocerans were numerous and constituted up to 75% of the total plankton, whilst in other samples collected above greater depths, copepods made up 60 to 85% of the total plankton. In the coral reef lagoon of south and southwest New Caledonia, typically hyponeustonic copepods (pontellids) often comprised 5% of the total copepod populations.     

Seston composition of the bottom waters of Great Lameshur Bay,St. John,US Virgin Islands

Seston composition [particulate organic carbon (POC), particulate nitrogen (PN), phyto- and microzooplankton numbers and biomass] was investigated in the bottom waters of Great Lameshur Bay, St. John, Virgin Islands (USA), in October, 1970 during Tektite II Mission 17–50. Mean values of 67.4 g POC · I^-1 and 7.2 g PN · I^-1 were determined. A mean phytoplankton carbon content of 42.2 g · I^-1 and zooplankton carbon content of 5.5 g · I^-1 were calculated from counts. The phytoplankton consisted mainly of dinoflagellates 71.2% phytoplankton carbon. Copepods were the dominant zooplankters (61.8% zooplankton carbon), followed by larvaceans (30.9% zooplankton carbon). Organic carbon content of counted zooplankton faecal pellets ranged between 0.4 and 1.6 g · I^-1, and amounted probably to about 15% of the total zooplankton carbon value. Plankton and detritus components as possible food for coral-reef animals are discussed. The ratio carbon: nitrogen of suspended particles is compared to that of sedimented matter.     

Estimating carbon flux to pelagic grazers in the ice-edge zone of the eastern Bering Sea

Zooplankton ingestion of phytoplankton carbon in the iceedge zone of the Eastern Bering Sea was measured using a deck incubation approach in 1982. Using further samples collected in 1983, the plant cell carbon to cell volume ratio was estimated at 0.0604 pg m^–3 from an experimentally determined particulate carbon to seston volume relationship. The application of this conversion to the results of experimental incubations of natural plant stocks with net-caught zooplankton produced ingestion rates of 68.8 and 10.26 mg C g^–1 grazer d^–1 for copepods and euphausiids, respectively. Extrapolating these rates to in situ zooplankton biomass at the edge of the seasonal ice pack yielded carbon flux rates through the zooplankton community ranging between 6.5 and 32.8 mg C m^–2 d^–1. This consumption amounted to less than 2% of the daily phytoplankton production in the ice-edge zone.     

Composition of the near-reef zooplankton at Heron Reef,Great Barrier Reef

Using a light trap, zooplankton was sampled at three stations at Heron Reef, Great Barrier Reef: (a) a typical patch reef in the Heron lagoon, (b) a site in 8 m water on the southern slope of Heron reef, and (c) a station approximately 300 m south of (b), in the open water of the channel between Heron and Wistari reefs. Samples were taken at the surface and on the substratum at the lagoon and reef-slope stations, and at 3 to 6 m depth at the open-water station. A total of 114 taxa, many recognized as species, were distinguished in the samples. Pronounced differences existed in abundance, diversity, and taxonomic composition of the samples obtained at different stations. Less pronounced differences existed between surface and substratum samples from the same station. Near-reef samples were more similar to one another than to open-water samples. Decapod larvae, amphipods, and cumaceans were all abundant in near-reef samples and very rare in open-water samples. Forams, isopods, mysids and polychaetes were common in near-reef samples, and rare or absent in open-water samples. Copepods were abundant in all samples but the near-reef samples contained predominantly different species than did samples from the open water. The near-reef fauna included 66 taxa which did not occur in open-water samples. Many of these were epibenthic rather than strictly planktonic in behaviour.     

Trophodynamic function of copepods,appendicularians and protozooplankton in the late summer zooplankton community in the Skagerrak

The study was carried out in the Skagerrak during late summer when population development in the pelagic cycle culminated in the yearly maximum in zooplankton biomass. The cyclonic circulation of surface water masses created the characteristic dome-shaped pycnocline across the Skagerrak. The large dinoflagellate Ceratium furca dominated the phytoplankton biomass. Ciliates and heterotrophic dinoflagellates were the major grazers and, potentially, consumed 43–166% of daily primary production. The grazing impact of copepods was estimated from specific egg production rates and grazing experiments. The degree of herbivory differed between species (14–85%), but coprophagy (e.g. feeding on fecal pellets) and ingestion of microzooplankton were also important. The appendicularian Oikopleura dioica was present in lower numbers than copepods, but cleared a large volume of water. The grazing impact of copepods and O. dioica was estimated to 57±24% and 12±12% of daily primary production, respectively. Sedimentation of organic material (30 m) varied between 169 and 708 mg C m^–2 day^–1, and the contribution from the mesozooplankton (copepod fecal pellets and mucus houses with attached phytodetritus of O. dioica) was 5–33% of this sedimentation. Recycling of fecal pellets and mucus houses in the euphotic zone was 59% and 36%, respectively. However, there was a high respiration of organic material by microorganisms in the mid-water column, and 34% of the sedimenting material actually reached the benthic community in the deep, central part of the Skagerrak.     

Size composition of particulate organic matter in the lagoon of Tikehau atoll (Tuamotu archipelago)

Suspended particulate matter was comprehensively investigated from 6 to 17 April 1986 in the lagoon of Tikehau atoll (15°00S; 148°10W). Dry weight (DW), particulate organic carbon (POC), adenosine triphosphate (ATP), and chlorophyll a were measured for five size-classes (0.2 to 0.8 m, 0.8 to 3 m, 3 to 35 m, 35 to 200 m, and 200 to 2000 m). Taxa were identified and counted for the whole plankton (both autotrophic and heterotrophic). Particles <3 m accounted for 81% of the total POC (192 mg m^-3), and detritus comprised 82% of the total POM. Phytoplankton (cyanobacteria plus algae) accounted for 35% of the living carbon, 75% of which consisted of heterotrophic bacteria and cyanobacteria. The zooplankton biomass was composed of 31% nano-, 26% micro-, and 43% mesoplankton.     

Detrital pathways in a coral reef lagoon

Coral reef lagoons are generally regarded as zones of net heterotrophy reliant on organic detritus generated in more productive parts of the reef system, such as the seaward reef flat. The abundance and biomass of sediment infauna were measured seasonally for one year (1986) within the lagoon of Davies Reef, central Great Barrier Reef, to test the hypothesis that macrofaunal biomass and production of coral reef lagoons would decrease with distance from the reef flat and would change seasonally. In general, there were no simple relationships between infaunal standing stock or production and distance from the reef flat or season. Bioturbation by callianassid shrimps negatively affected the abundance of smaller infauna, suggesting a community limited by biogenic disturbance rather than by supply of organic material. Polychaetes and crustaceans were dominant amongst the smaller infauna (0.5 to 2mm) while larger animals (> 2 mm) were mostly polychaetes and molluscs. Mean biomass of infauna at both sites and all seasons was 3 181 mg C m^?2. The smaller animals (0.5 to 2 mm) contributed about 40% of total macrofaunal respiration and production although they represented only 15% of the total macrofaunal biomass. The biomass of macrofauna was about equal to that of the bacteria and meiofauna, while respiration represented 10 to 20% of total community respiration. Consumption by macrofauna accounts for only 3 to 11% of total organic inputs to sediment, with a further 14 to 17% being lost by macrofaunal respiration.     

Plankton community dynamics of the central Great Barrier Reef Lagoon: Analysis of data from Ikeda et al.

Plankton data collected by Ikeda et al. (1980) from the central region of the Great Barrier Reef, and spanning two years (1976 through 1978) of zooplankton records, have been analyzed extensively for spatial and temporal patterns. Estimates of net zooplankton (including chaetognaths, copepods, and larvaceans) and microzooplankton (juvenile copepods, encompassing nauplii and copepodites, and ciliates) were assessed at three stations across the 60 km lagoon. Temperature, salinity, and chlorophyll a were also measured. A cross-lagoonal gradient was identified in the plankton, concurring with results of related surveys of benthic taxa, such as scleractinian corals, soft corals, macro-algae, fish, sponges, crinoids, etc. Two associations of net zooplankton were identified. The first was associated primarily with the inner lagoon; the second with the outer lagoon. The inshore association was characterized by higher abundances of almost all net zooplankton taxa, particularly chaetognaths, copepods, polychaetes, decapods, and meroplanktonic larvae as well as higher concentrations of chlorophyll a. This inshore association wove back and forth across the lagoon through time, dominating the lagoon entirely during periods of high river discharge, reaching the mid-shelf platform reefs in this region, and sometimes being entirely absent during dry periods. Both seasonal and annual peaks in plankton abundance were generally linked with degree of runoff. Summer/autumn peaks of abundance were evident in chaetognaths, copepods, and larvaceans while annual variation was detected in the former two as well as in chlorophyll a concentrations. Depth stratification was noted in juvenile copepods and chlorophyll a concentrations at the center of the lagoon, with higher abundances recorded in deeper waters. The central Great Barrier Reef lagoon was found to be typical of other tropical coastal waters where plankton community dynamics are controlled primarily by physical factors. We suggest that any substantial changes in river discharge in this area will affect plankton production.A.I.M.S. Contribution No. 242     

Ecological studies of the coral predator Acanthaster planci in the south pacific

Ecological surveys involving over 500 man-days between 1966 and 1969 indicate that the coral-eating sea star, Acanthaster planci, is a normal component of the coral reef community throughout the tropical Pacific, and that its abundance in the past has probably been underestimated. The sea star is not uncommon in certain environments, particularly back-reef and lagoon slopes. Sheltered, inner reefs are generally preferred over less protected reefs. Recently reported population explosions of A. planci at Guam and on the Great Barrier Reef of Australia appear to be isolated, widely-separated, local infestations of unknown cause. The infestation on the Great Barrier Reef has not spread beyond the area off Cairns and Innisfail. Approximately 40 of the more than 1000 reefs comprising the Great Barrier Reef complex have been infested heavily.     

Resident mysids: community structure,abundance and small-scale distributions in a coral reef lagoon

Seasonal and diel variations in community structure and abundance of coral-reef lagoon mysids were examined at Davies Reef in the central region of the Great Barrier Reef (GBR) between June 1980 and May 1981. Twenty-five mysid species belonging to three subfamilies of the family Mysidae were captured during the study, with six new records for the GBR. The epibenthic mysid community differed from that in the overlying water, was faunistically uniform, but formed characteristic seasonal and diel groupings. The dominant epibenthic species were Erythrops sp., Anisomysis pelewensis, Doxomysis littoralis, A. laticauda, Prionomysis stenolepis, A. lamellicauda, and A. australis, five of which formed schools. Total mysid abundances ranged between 110 and 790 m^-3 with peak abundance in October. Schooling species occurred at local densities of up to 500 000 m^-3. Mysids were absent from shallow and midwater depths during the day, but were distributed throughout all depths at night with peak abundances in mid-water and deep layers. The dominant species in the water column at night were Pseudanchialina inermis, A. laticauda and Gastrosaccus indicus, in descending order of abundance. Lagoonal mysids contribute little to the food of sessile reef planktivores, as all but three species remain concentrated near or on the lagoon floor both day and night. The contribution of resident lagoon mysids to reef trophodynamics is probably through remineralization of lagoon detritus. Given the vast reef areas comprised of sandy lagoons, the large populations and relatively large size of lagoon mysids, this trophodynamic role may be of considerable importance.A.I.M.S. Contribution No. 477     

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.