Structural-functional relationships in the pelagic community of the eastern tropical Atlantic Ocean

Sampling was carried out in the eastern tropical Atlantic duringJuly–September, 1987 on four transects at latitudes 6,12, 18 and 24°W, from 6°S to 4°N. A total numberof 81 stations were occupied where physical, chemical and biologicalmeasurements were taken. Temperature, salinity, chlorophylla, phytoplankton, primary production (PP), bacterial production,micro-, meso-, macrozooplankton and mesopelagic fish and squidwere sampled in the upper 120 m layer. The biomass size spectrumof the plankton community was relatively flat within the divergencezone but the slope of the size spectrum increased towards theconvergence zone meaning an increased contribution of the small-sizedorganisms. Higher turnover rates (the ratio of PP to the totalcommunity biomass) were observed in regions where the size spectrasloped more steeply. PP exceeded the community metabolism bythree to five times in regions where the size spectra were sharplysloped. In regions with flat-type spectra (with slopes from-0.2 to 0) a balance between PP and community metabolism wasobserved. The variation coefficient and the dispersion index(the variance to mean ratio) indicated an increase of the spatialvariability of biomass in the sequence: ‘phytoplankton–mesozooplankton–macrozooplankton–micronekton’.At the higher trophic level of the zooplankton community, betweenthe biomass of squid and gelatinous zooplankton, a logarithmiclinear relationship was derived. Squid actively consumed gelatinousorganisms in regions of high concentrations of their prey, althoughlarge copepods, euphausiids, amphipods, flying fish, and myctophidfish also contributed to their diet.     

Plankton composition, abundance and dynamics in a severely stressed cooling reservoir

The effects of severe thermal stress imposed by the intermittentoperation of a nuclear reactor on plankton abundance and dynamicswere investigated in Pond C, a cooling reservoir on the SavannahRiver Site in South Carolina, USA. Temperatures in Pond C rangedup to 58°C during reactor operation. The thermal effluenteliminated zooplankton from regions where the temperature exceeded45°C, reduced zooplankton abundance by 1–3 ordersof magnitude and typically halved the number of taxa. Reactoroperation also reduced phytoplankton biovolume, often by >70%.During intermittent reactor operation, the rotifer Filinia longisetadominated the zooplankton and two cladocerans of the genus Moinawere abundant. These species were not abundant during extendedreactor shutdowns. The success of Filinia and Moina was dueprimarily to their tolerance of high temperatures. Sparse phytoplanktonprobably limited some zooplankton taxa, although other taxa,such as Filinia may have utilized bacterial resources. Reactoroperation may have intensified predation on crustacean zooplanktonwhen fish were concentrated in refuge areas with zooplankton.Processes by which zooplankton repopulated the reservoir afterreactor shutdown were inferred from zooplankton distributionpatterns, and population growth and birth rates. Repopulationtypically occurred within a few days due to rapid growth ofpopulations from refuge areas within the reservoir and colonistsbrought in through a tributary canal. Mechanisms of zooplanktonrepopulation in Pond C suggest that refuges or colonizationcorridors should be maintained when the re-establishment ofcommunities following cessation of stresses is desired.     

Habitat loss effects on spatial distribution of non-vascular epiphytes in a Brazilian Atlantic forest

The vertical distribution of the richness, abundance, and composition of epiphytic bryophytes was studied in a matrix of fragmented habitats in the Brazilian Atlantic rainforest of the Murici Ecological Station (9°11′05″–9°16′48″ S, 35°45′20″–35°55′12″ W), northeastern Brazil. The aim was to compare the horizontal (between sites) and vertical (between phorophyte heights) turnover to test a hypothesis based on niche width vulnerability. There was a highly significant decreasing of richness accompanying the loss of habitat, and the most conserved fragment housed a total richness more than 10 times higher than the less conserved fragment. Epiphytes failed to colonize lower trunks (2.1–10 m) and higher zones in most of the non-conserved fragments; they were restricted to the base (0–2 m) and displayed a clear altered floristic composition. The species with restricted ecological amplitudes such as sun and shade tolerant taxa were more negatively affected by habitat loss than generalists. Although the mean richness of generalists decreased in non conserved fragments, the proportional contribution of this guild increased, proving that these taxa are the ones which persist in disturbed sites. The forest fragments capable of harboring rich epiphyte flora in the area studied are over 300 ha in size, which is far from being a common size among Brazilian Atlantic rainforest fragments. Hence, our results highlight the need of conserving the few large remnants in this ecosystem.     

Heterotrophic protozoa and small metazoa: feeding rates and prey-consumer interactions

A common approach to divide zooplankton into groups has beenby size or size fractionation (micro-, meso- and macrozooplankton).Whereas almost all zooplankton retained by 200 µm meshare metazoa, those not retained are proto- and metazoa. Evenso, the variability of major taxa among those retained by 200µm mesh can range widely between samples, that of passing200 pm can vary even more when considering the grazing impact.If heavily weighted towards protozoa, the <200 µm communityfeeding rate on small phytoplankton could be several times therate when most animals would be metazoa. Also, the interactionbetween proto- and metazooplankton passing 200 µm meshought to be considered, as should be that among protozoa. Usingpublished data from the North Atlantic Ocean, the potentialimpact of small metazooplankton on the chlorophyll standingstock and primary productivity as well as on protozooplanktonwas evaluated. It was found that metazoo plankton passing <200µm mesh removed a much larger part of the primary productivitythan those retained by 200 µm mesh. Although the biomassof the 200 µm mesh metazoa was close to that of protozoapassing the same mesh, their ration was only a relatively smallpart of the primary productivity ingested by the latter. Yet,due to their unusually high abundance in these oceanic waters,the overall metazooplankton appeared to come close to controllingprotozooplankton >50 µm³ in volume, i.e. those whichcould be actively perceived. It is hypothesized that in theabsence of control by meta zooplankton, protozoa control theirown abundance by predation/cannibalism.     

Continuous plankton records: seasonal variations in the distribution and abundance of plankton in the North Atlantic Ocean and the North Sea

Charts are presented of the seasonal variations in the distributionof four phytoplankton and five zooplankton taxa in the NorthAtlantic and the North Sea. The main factors determining theseasonal variations appear to be the distribution of the mainoverwintering stocks, the current system and, in some instances,temperature control of the rate of population increase. Informationis presented about the variation with latitude (over the rangefrom 34° N to 65 ° N) of the seasonal regime of theplankton. On the assumption that there is a relationship betweennutrient supply and vertical temperature stratification themain features of this variability can be interpreted. In thesouth (to about 43° N) nutrient limitation plus grazingappear to be dominant, resulting in a bimodal seasonal cycleof phytoplankton. North of about 60° N the system appearsto be limited by the size of the phytoplankton stocks beinggrazed primarily by Calanus Finmarchicus and Euphausiacea. Inan extensive zone, from about 44° N to 60° N, it wouldappear that the spring bloom of phytoplankton is under-exploitedby grazing while in summer the zooplankton graze the daily productionof the phytoplankton, the stocks of which are probably maintainedby in situ nutrient regeneration. The implications, for at leastthis mid-latitude zone, that rates and fluxes of processes,as opposed to density dependent interactions between stocks,play a major role in the dynamics of the seasonal cycle is consistentwith previously reported observations suggesting that physicalenvironmental factors play a major role in determining year-to-yearfluctuations in the abundance of the plankton.     

Using neural networks to predict surface zooplankton biomass along a 50{degrees}N to 50{degrees}S transect of the Atlantic

Four Atlantic transects between the UK and the Falkland Islandswere carried out during spring and autumn as part of the AtlanticMeridional Transect (AMT) programme. These 50°N to 50°Stransects cross several ocean regions. An optical plankton counter(OPC-1L) sampled continuously along the transects from the ship'suncontaminated seawater supply, giving a surface distributionof zooplankton abundance and size. Measurements of underwayfluorescence-derived chlorophyll, sea surface temperature andsalinity were also taken from the uncontaminated seawater supply.The relationship between zooplankton biomass and these variableswas investigated using multiple linear regression and neuralnetwork techniques. In the analysis, log_e-transformed biomasswas used to reduce the influence of extreme values. Two transectswere used to develop the models, and two to test the generalizationcapabilities of the models. Multiple linear regression couldexplain up to 55% of the observed variation in the transformedbiovolume, and demonstrated the association of hydrographicvariables and diel migration within the surface zooplanktoncommunity. Neural networks, starting with the same set of variables,were able to explain up to 78% of the variability, showing anincreased performance over the multivariate analysis. An optimizedmodel accounted for 77% of the variance in the original data.However, it showed greater generalization capabilities (R² =0.47) when applied to new data sets than either the originalneural network model (R² = 0.31) or the multiple linear regressionmodel (R² = 0.34). This study highlights the non-linear natureof the parameters' associations with the zooplankton biomassand their variability between oceanographic regions.     

The Influence of Ekman Transport on Zooplankton Biomass Variability off Southern Brazil

Cross- and long-shelf Ekman transport components were correlatedwith the variability of zooplankton biomass from 1977 to 1990off the southern Brazilian coast (27°S to 35°S and 46°Wto 54°W). During warm months the dominant cross-shelf Ekmantransport was oriented offshore, where an increase in zooplanktonbiomass occurred with intervals of 2–3 months. The presenceof the South Atlantic Central Water (SACW; the water mass thattypically upwells at the continental shelf) was confirmed byseasonal temperature–salinity diagrams. The high zooplanktonbiomass values observed in this study were frequently causedby salps, which play an important ecological role in the pelagiccommunity in upwelling zones that has not been investigatedyet.     

Spatial distributions of copepod genera along the Atlantic Meridional Transect

The distribution of copepod taxa at a basin scale was analysed using three Atlantic transects (U.K. – Malvinas 1997, Malvinas – U.K. 1997, and South Africa – U.K. 1998). Integrated 200 m to surface zooplankton samples were taken daily, using WP2 nets (200-m mesh). The zooplankton were size-fractionated and sub-samples taken for carbon analysis. The remainder of the samples was preserved for taxonomic analysis of copepod genera. Multidimensional scaling (MDS) was used to identify zoogeographic regions from the copepod genera. Seven regions were identified: northern temperate, northern subtropical, equatorial, southern tropical, southern sub-tropical, southern temperate and Benguela upwelling. Analysis of similarity showed that most regions were significantly different from each other except: northern temperate and southern temperate, northern temperate and southern subtropical, and northern subtropical and southern subtropical. The genera significant in determining the regions were identified. These regions were compared to other schemes of biological and hydrographic areas. The MDS also showed that the copepod composition in the tropical and subtropical regions was less variable than the temperate and Benguela stations. Latitudinal trends in diversity and size were also investigated. Copepod genera showed a reduction in richness at higher latitudes. Copepod size did not show any substantial or consistent change with latitude along these transects, as demonstrated by both the numerical abundances in each size category, and the carbon biomass per individual. The proportion in each size fraction was quite uniform over the transect.     

Tropical zooplankton in the highly-enclosed lagoon of Taiaro Atoll (Tuamotu Archipelago, French Polynesia)

 Nocturnal zooplankton assemblages around Taiaro Atoll were sampled over six nights during February 1994. Replicate zooplankton samples were collected at windward and leeward locations in the enclosed lagoon and adjacent ocean with a metered net (85 cm diameter, 500 μm mesh) towed for 15 min at 5 m depth. The zooplankton community in the lagoon was very different from that in the ocean. Oceanic samples contained 50 mostly holoplanktonic taxa (diversity index, H′=2.62; evenness index, J′=0.67). Lagoonal samples contained 19 mostly meroplanktonic taxa (H′=1.54, J′=0.52) with three taxa (decapod larvae; an ostracod, Cypridina sp.; a copepod, Acartia fossae) contributing >90% of the individuals. Unlike the ocean, zooplankton distributions in the lagoon were not homogenous; instead spatial patterns were apparently formed by the interaction between hydrodynamic processes and species-specific behaviour. Accepted: 28 August 1997     

Composition,size, and biomass of zooplankton in large productive Florida lakes

Crustacean zooplankton data were compiled from long-term observational studies at seven large shallow Florida lakes, to determine whether there are general characteristics in regard to species composition, body size, and biomass. In particular, we examined whether patterns in body size and species richness fit empirical models developed by Stanley Dodson. The lakes included range in size from 125 to 1730 km² and encompass mesotrophic to hyper-eutrophic conditions. We found that zooplankton biomass was strongly dominated by one species of calanoid copepod—Arctodiaptomus dorsalis. Large daphnids were absent, and Cladocera assemblages were dominated by small taxa such as Ceriodaphnia, Chydorus, and Eubosmina. The total number of species of pelagic cladocerans (8–12) was consistent with Dodson’s predictions based on lake area. The average size of crustacean zooplankton in Florida lakes is small in comparison with temperate communities. A. dorsalis is the smallest calanoid copepod in North America, and the mean length of Cladocera (0.6 mm) is consistent with Dodson’s results that size decreases from temperate to tropical zones. Total biomass of crustacean zooplankton was very low, ratios of zooplankton to phytoplankton biomass (0.01–0.1) are among the lowest reported in the literature, and the zooplankton displayed short-lasting early spring peaks in biomass. Cladocera were almost entirely absent in spring and summer. Factors known to occur in Florida lakes, which appear to explain these characteristics of biomass, include intense fish predation and high summer water temperature.     

The aggregation of polar cod (Boreogadus saida) in the deep Atlantic layer of ice-covered Amundsen Gulf (Beaufort Sea) in winter

During the Circumpolar Flaw Lead System Study (CFL, 2007–2008), large aggregations of polar cod were detected in winter in the Amundsen Gulf (Western Canadian Arctic) using the EK60 echosounder of the CCGS Amundsen research icebreaker. Biomass estimated over 10 months reached a maximum of 0.732 kg m⁻² in February. Aggregations were encountered only in the presence of an ice cover from December to April. The vertical extent of the aggregations was dictated by temperature and zooplankton prey distribution. In winter, polar cod generally occupied the relatively warm deep Atlantic Layer (>0°C), but a fraction of the densest aggregations occasionally followed zooplankton prey up into the cold Pacific Halocline (−1.6 to 0°C). The diel vertical migration of polar cod was precisely synchronized with the seasonally increasing photoperiod. Throughout winter, polar cod aggregations migrated to progressively deeper regions (from 220 to 550 m bottom depths) in response to increasing light intensity, presumably to avoid predation by visual predators such as the ringed seal. Comparing Amundsen Gulf and Franklin Bay indicates that the entrapment of polar cod in embayments during winter is an important mechanism to provide marine mammal predators with dense concentrations of their main prey within their diving range.     

Larger zooplankton in Danish lakes after cold winters: are winter fish kills of importance?

Winter fish kills can be intense under ice in shallow lakes, and have cascading effects on the food web and ultimately on lake water clarity. In maritime Western Europe, winters are usually mild, but occasional colder periods may also have strong effects on lake fish communities. Global warming may have disproportionate effects by delaying freezing and shortening the period of ice coverage. We studied differences in zooplankton (cladocerans, copepods, and rotifers): phytoplankton biomass, zooplankton community structure, and individual body size among 37 Danish lakes of various depths, chemical characteristics, and trophy, by comparing four winters of different severity (mean winter temperatures ranging from −1.19°C in 1996 to +2.9°C in 1995). We found that crustacean mean body sizes were significantly larger in the summer following a severely cold winter. The zooplankton communities in the summer after a cold winter had a significantly larger proportion of larger-bodied species and taxa. Phytoplankton biomass, expressed as chlorophyll-a (chl-a), was lower and zooplankton herbivory (chl-a:TP index), higher, in the summer after the severely cold winter of 1995/1996. All these effects were stronger in shallow lakes than in deep lakes. Changes in zooplankton during summer 1996, compared with other years, were likely caused by fish kills under ice during the preceding severe winter of 1995–1996. Fish kills due to under ice oxygen depletion would be expected to occur earlier and be more complete in the shorter water columns of shallow lakes. With climate change, severe winters are predicted to become less frequent and the winters to be milder and shorter. In general, this is likely to lead to higher winter survival of fish, lower zooplankton grazing of phytoplankton the following summer and more turbid waters, particularly in shallow eutrophic lakes.     

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16–36 μm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28–78 μm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 μm) “highly edible” algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations. Received: 31 March 1998 / Accepted: 19 October 1998     

The Iberian Peninsula as a potential source for the plant species pool in Germany under projected climate change

The application of niche-based modelling techniques to plant species has not been explored for the majority of taxa in Europe, primarily due to the lack of adequate distributional data. However, it is of crucial importance for conservation adaptation decisions to assess and quantify the likely pool of species capable of colonising a particular region under altered future climate conditions. We here present a novel method that combines the species pool concept and information about shifts in analogous multidimensional climate space. This allows us to identify regions in Europe with a current climate which is similar to that projected for future time periods in Germany. We compared the extent and spatial location of climatically analogous European regions for three projected greenhouse gas emission scenarios in Germany for the time period 2071–2080 (+2.4°C, +3.3°C, +4.5°C average increase in mean annual temperature) to those of the recent past in Europe (1961–90). Across all three scenarios, European land areas which are characterised by climatic conditions analogue to those found in Germany decreased from 14% in 1961–1990 to ca. 10% in 2071–2080. All scenarios show disappearing current climate types in Germany, which can mainly be explained with a general northwards shift of climatically analogous regions. We estimated the size of the potential species pool of these analogous regions using floristic inventory data for the Iberian Peninsula as 2,354 plant species. The identified species pool in Germany indicates a change towards warmth and drought adapted southern species. About one-third of the species from the Iberian analogous regions are currently already present in Germany. Depending on the scenario used, 1,372 (+2.4°C average change of mean annual temperature), 1,399 (+3.3°C) and 1,444 (+4.5°C) species currently not found in Germany, occur in Iberian regions which are climatically analogous to German 2071–80 climate types. We believe that our study presents a useful approach to illustrate and quantify the potential size and spatial distribution of a pool of species potentially colonising new areas under changing climatic conditions.     

Thermal ecotypes of amphi-Atlantic algae. II. Cold-temperate species (Furcellaria lumbricalis andPolyides rotundus)

Two species of cold-temperate algae from the North Atlantic Ocean,Polyides rotundus andFurcellaria lumbricalis, were tested for growth and survival over a temperature range of −5 to 30 °C. In comparisons of eastern and western isolates, bothF. lumbricalis, a North Atlantic endemic, andP. rotundus, a species having related populations in the North Pacific, were quite homogeneous.F. lumbricalis tolerated −5 to 25°C and grew well from 0 to 25°C, with optimal growth at 10–15 °C.P. rotundus tolerated −5 to 27°C, grew well from 5 to 25°C, and had a broad optimal range of 10–25°C. Both species tolerated 3 months in darkness at 0°C. In neither case could any geographic boundary be explained in terms of lethal seasonal temperatures, suggesting that these species are restricted in distribution by strict thermal and/or daylength requirements for reproduction. The hypothesis that northern species are more homogeneous than southern taxa in terms of thermal tolerance was supported. A second hypothesis, that disjunct cold-temperate species should be more variable than pan-Arctic species, was not supported.     

Plasticity in filtering screens of Daphnia cucullata×galeata hybrids and parental species at two food concentrations

The coexistence of Daphnia cucullata × galeata hybrids with the parental species D. galeata and D. cucullata was investigated by measuring areas and mesh sizes of filtering structures of these herbivorous zooplankton taxa cultivated at low and high food concentrations. The clearance rates and somatic growth rates were also determined. When reared at low food concentration, all taxa had larger filtering areas. Larger filtering areas also resulted in higher clearance rates. Differences between taxa in both filtering area and clearance rate were caused mainly by interspecific size differences. Hybrids had the largest absolute mesh sizes, and the parental species had smaller mesh sizes. Hybrids also showed heterosis in somatic growth rate at high food concentration. The observed taxon-specific differences in mesh size and somatic growth rate contribute to resource partitioning between the taxa and thus to their successful coexistence in lakes. Received: 1 March 1999 / Accepted: 25 May 1999     

Mesh size and collection characteristics of 50-cm diameter conical plankton nets

This paper compares collection characteristics of #2-(363 µm), #10-(156 µm), and #20-(76 µm) mesh conical plankton nets: dimensions were 50-cm diameter by 1.6-m long. The #2-mesh net severely underestimated the abundances of Lake Michigan copepods and cladocerans with the exception of the largest species (Limnocalanus macrurus). Zooplankton abundance estimates were more similar for the #10- and #20-mesh nets collections. Nauplii, however, were severely undersampled by the #10-mesh net with abundance estimates approximately 8 to 12 times lower than for the #20-mesh net collections. Most other larger zooplankton were 50% more abundant in the 20-mesh net collections than in the #10-mesh net collections: such consistent differences occurred despite large variations in taxa size. This indicates that a sampling bias occurred other than the loss of zooplankton through the meshes of the #10 net. We hypothesize that, by incorrectly locating the flowmeter in the mouth of the plankton net, we underestimated the volume of water filtered by the easily-clogged #20-mesh net and therefore overestimated taxa abundances. We conclude that the #10-mesh net provided accurate estimates of microcrustacean zooplankton abundances except for nauplii. The #10-mesh net used in our study had a filtration area ratio of 3.06 and operated at a calculated average filtration efficiency of 98%. The #20-mesh net had a filtration area ratio of 1.86 and operated at calculated average filtration efficiencies ranging from 64.7% (41.7 m station) to 79.6% (6.3 m station). Calculations are presented which show how the filtration efficiencies of the nets used in our study could be improved by net redesign.     

Spatio-temporal variations of the zooplankton abundance and composition in a West African tropical coastal lagoon (Grand-Lahou,Côte d’Ivoire)

Zooplankton constitutes a sensitive tool for monitoring environmental changes in coastal lagoons; however, the available information on zooplankton communities is not sufficient to optimize their rational management. The relationships between zooplankton distribution and environmental factors were studied in a tropical lagoon to test whether the indicator properties of zooplankton assemblages could be used to monitor water quality, in a context of expected eutrophication provoked by an increasing anthropogenic activity. Twenty-one (21) stations were sampled monthly from January to December 2004. The community was composed of 65 taxa including Copepoda, Rotifera, and Cladocera. Copepoda was the most abundant group (81% of total numbers). The main zooplankton species were Oithona brevicornis, Acartia clausi, and Brachionus plicatilis. The highest zooplankton abundance (171–175 ind. l⁻¹) was recorded during the long, dry season (February–April) and the lowest (40–45 ind. l⁻¹) during the rainy and the flood periods (June–July). At a spatial level, the lowest abundance was observed in the estuarine zone. During the dry seasons (December–April and August–September), marine zooplankton taxa were abundant near the channel of Grand-Lahou, and brackish water taxa dominated in the other sites. Multivariate analyses (Co-inertia) showed that the composition of zooplanktonic communities and their spatio-temporal variations were mainly controlled by salinity variations closely linked to the climatic and hydrological context. The role of the trophic state on zooplankton communities could not be clearly evidenced. Our results and a comparison with previous studies in the neighboring, highly polluted Ebrié Lagoon suggest that the ratio between Oithona and Acartia abundance could be used as biological indicator for the water quality. Handling editor: S. I. Dodson     

Seasonal changes in zooplankton composition in the Barents Sea, with special attention to Calanus spp. (Copepoda)

The Barents Sea is an important area with respect to fisheriesresources (i.e. capelin and cod). In May, June and August 1981zooplankton biomass was measured along a transect at 30°E,from the ice border southwards. A maximum was recorded in Atlanticwater by the end of June (>100 g wet weight m^–2 InAugust the biomass values were relatively low south of the Polarfront and increased northwards into Arctic water (–50g m^–2 The species composition was influenced by the distributionof cold Arctic water and warmer Atlantic water. The zooplanktonwas dominated by the copepods Calanus finmarchicus and C. glacialis;the former is regarded as an Atlantic species and C. glacialisas an Arctic species.     

Crustacean zooplankton community structure in temporary and permanent grassland ponds

Lentic community structure varies across a size gradient of ponds and lakes with physical factors, such as pond drying, and biotic factors, such as fish predation, determining the species assemblage. We studied the effects of pond drying and fish absence on crustacean zooplankton across a gradient of pond sizes in a Texas grassland. We determined the species compositions and size distributions of crustacean zooplankton in 20 temporary and 18 permanent ponds in April after March rains had refilled the ponds. The surface areas of temporary and permanent ponds ranged from <0.01 to 0.21 ha and 0.04 to 13.8 ha, respectively, and temporary ponds were significantly smaller, on average, than permanent ponds. Fish were absent from all temporary ponds and present in all permanent ponds. We detected a difference in the zooplankton species assemblages of the temporary and permanent ponds. Out of 14 total zooplankton taxa that occurred in eight or more ponds, seven taxa were significantly more prevalent in temporary ponds and four taxa were significantly more prevalent in permanent ponds. The sizes of zooplankton in the temporary fishless ponds were greater than those in the permanent ponds with fish present. We concluded that pond size mediated susceptibility to pond drying, and pond drying determined the presence and absence of fish and their secondary trophic-level effect on zooplankton community structure. Handling editor: Steven Declerck