Vertical structure of small metazoan plankton, especially noncalanoid copepods. I. Deep Arabian Sea

The abundance and vertical distribution of micro-metazoans sampledwith fine nets of 0.05 mm mesh size were studied at three stationsin the Arabian Sea during the intermonsoon period (April/May1987) and down to 1850 m depth. In the epipelagic zone (0–100m). values of biomass and metazoan abundance tended to be higherthan those reported for other tropical oceanic areas. In themesopelagic zone, which is characterized by an extreme oxygendeficiency between 100 and 1000 m depth, the abundance of metazoantaxa and species numbers of non-calanoid copepods were largelyreduced. However, intermediate abundance maxima occurred withinthis zone, which were dominated by specific metazoan taxa (copepods.appendicularians) and species of non-calanoids (Oncaea sp. C).In the bathypelagic zone below 1050 m, the species diversityof the dominant copepod family Oncaeidae increased substantially.Two-thirds of a total of 69 oncaeid species recovered were confinedto this layer. As most of them were small in size and occurredin low abundance only, the increase in total oncaeid densityand/or plankton biomass was less conspicuous. Dominant Oncaeaspecies in the bathypelagic zone were O.longipes and O.brodskii.The results are compared with published data from the ArabianSea and other tropical oceanic areas with and without an extrememesopelagic oxygen minimum zone Possible causes of the intermediateabundance maxima within the oxygen deficiency zone are discussed.     

Life cycle of Cyclops vicinus in relation to food availability, predation, diapause and temperature

The seasonal cycle of Cyclops vicinus was studied during a 5year period in the shallow and hypertrophic Lake Søbygård.The annual number of generations varied between three and sixand sometimes included midsummer generations. Naupliar and copepoditedensity was extremely high, the maximum recorded being 1313and 745 1^–1 respectively. The seasonal fluctuation inthe abundance of C.vicinus was related to the concentrationof edible phytoplankton (EDP): when diatoms and cryptomonadswere abundant, nauplii and copepodites were also abundant, whereaswhen greens such as Scenedesmus spp. were dominant, no cohortswere observed. The maximum density of copepodites in the separatecohorts and the amount of EDP, expressed as carbon content,were found to be significantly correlated. The development ofthe second cohort also seemed to depend on the occurrence ofa peak in rotifer density. High number of eggs per female generallycoincided with high biovolume of EDP, suggesting that algaeconstituted a major part of the diet of adult C. vicinus. Theduration of the naupliar stage was inversely related to temperatureand is one of the shortest times that have been recorded inthe literature. The short duration is probably attributableto the abundance of food. Fish composition and the calculatedsex ratio of C.vicinus indicate low predation pressure fromplanktivorous fish. The high density of C.vicinus during thesummer, when the whole population would be expected to haveentered diapause, is probably attributable to the concurrenceof low predation and the abundance of food.     

Periodicity of composition, abundance, and vertical distribution of summer zooplankton (1977/1978) in Ezcurra Inlet, Admiralty Bay (King George Island, South Shetland)

Seasonal succession and variation in species composition, density,biomass, age distribution and frequency of zooplankton (mainlythe Copepoda) were analysed during the austral summer of of1977/1978 in Ezcurra Inlet, a part of the Antarctic coastalecosystem. Small zooplankters (i.e., cyclopoids of the generaOncaea and Oithona, and calanoids Drepanopus pectinatus andScolocithricella glacialis) were found to predominate in termsof abundance and percentage contribution. The zooplankton biomasswas dominated by larger organisms of the Metrididae and Calanidae(Calanoida). The maximum abundance and maximum biomass of copepodswere recorded in February; two small peaks in copepod biomassbeing observed in late December and late January, and a lesserbiomass peak in late December. The vertical distribution ofcopepods in terms of their diel and seasonal (December, January,February, March) changes showed a day-time maximum to have occurredin the near-bottom layer, the nocturnal distribution being bimodalwith peaks within 0–10 m and 25–50 m. The summerzooplankton community in Ezcurra Inlet is controlled by trophic(phytoplankton composition and density) and hydrological (waterexchange with Bransfield Strait) conditions prevailing in thearea.     

Distribution and ecology of tintinnids in the plankton of Lebanese coastal waters (eastern Mediterranean)

The tintinnids of Jounieh Bay in Lebanon were identified andtheir numerical abundance determined in horizontal and verticalnet samples from February 1979 to December 1980. Large fluctuationsin abundance and composition of the tintinnid fauna occurredduring the period of investigation. Two distinct peaks werenoted, a major one in May–June and a minor one in November–December.The first followed the main phytoplankton peak, suggesting apositive nutritional relationship between the two populations.The most abundant species were Tintinopsis beroidea and Eutintinnuslusus-undae. A few other species showed great abundance duringshorter periods. In all, 121 species were recorded during thisperiod, and of these 12 belong to the genus Tintinnopsis and11 to Eutintinnus.     

Summer coastal zooplankton biomass and copepod community structure near the Italian Terra Nova Base (Terra Nova Bay, Ross Sea, Antarctica)

The structure of the zooplankton biotic community and of copepodpopulation in the coastal area of Terra Nova Bay (Ross Sea,Antarctica) was investigated during the 10th Italian AntarcticExpedition (1994/1995). Zooplankton biotic community consistedmainly of pteropods (Limacina helicina and Clione antarctica),Cyclopoid (Oithona similis), Poecilostomatoid (Oncaea curvata)and Calanoid (Ctenocalanus vanus, Paraeuchaeta antarctica, Metridiagerlachei and Stephos longipes) copepods, ostracods, larvalpolychaetes and larval euphausiids. Zooplankton abundance rangedfrom 48.1 ind m^–3 to 5968.9 ind m^–3, and copepodabundance ranged from 45.2 ind m^–3 to 3965.3 ind m^–3.The highest peak of zooplankton abundance was observed between25 m and the surface and was mainly due to the contributionof O. similis, O. curvata and C. vanus. Zooplankton biomassranged from 5.28 mg m^–3 to 13.04 mg m^–3 dry weight;the maximum value was observed between 25 m and the surface.Total lipid content varied from 216.44 to 460.73 mg g^–1dry weight.     

Ammonium dependency in Closterium aciculare T.West, a planktonic desmid from alkaline, eutrophic waters

One Japanese and some Dutch clones of the planktonic desmidspecies Closterium actculare were unable to grow on nitrateas the nitrogen source whereas they did grow well on ammoniumLack of nitrate reductase activity in the absence of ammoniumsuggests a structural shortage in the synthesis of this enzyme.Distributional patterns of C aciculare in space and time, ie. in eutrophic lakes, with abundance peaks in periods of circulationof the water column, may be well explained from the above-mentionedphysiological characteristic     

Ontogeny and distribution of Hygophum benoiti(Pisces, Myctophidae) of the North Western Mediterranean

Two species of the lanternfish genus Hygophum, H.benoiti andH.hygomii, are present in the Mediterranean. Differentiationof the larvae from these two species has been problematic. Anattempt to identify such larvae in the plankton samples of theNorth Western Mediterranean led to a broader study that includedthe examination of a large number of plankton samples collectedin the region since 1976. The large number of Hygophum larvaeobtained during July 1992 in the North Western Mediterranean(that proved to be H.benoiti), together with collections oflarvae from different geographic areas (especially H.hygomiifrom the Southwest Indian Ocean), allowed a detailed morphometriccomparison of the larvae of H.benoiti and H.hygomii. Developmentalstages of H.benoiti from yolk sac to post-flexion larvae aredescribed. Information on the temporal occurrence of H.benoitiin the plankton, as well as the spatial distribution of thisspecies during the survey of peak abundance (June/July 1992)in the North Western Mediterranean, is presented.     

Sources of seasonal variability in mesozooplankton aspartate transcarbamylase activity in coastal waters off Plymouth, UK

Some investigators have proposed aspartate transcarbamylase(ATCase) activity as an overall index of mesozooplankton productivity.However, seasonal changes in mesozooplankton species compositionhave never been investigated as a possible source of variationin ATCase activity. In this study, we investigate mesozooplanktoncomposition in terms of (i) developmental stages, (ii) speciesand developmental stages body mass and (iii) species composition,and their relationship to ATCase activity. In controlled laboratoryconditions, ATCase activity variability was closely relatedto changes in somatic growth rate of the copepod Calanus helgolandicus,but was not related to changes in nucleic acid concentrations.It can be argued, however, that the activity of this enzymeis partially involved in copepod somatic productivity, and shouldbe a good index of embryogenesis. In addition, changes in ATCaseactivity were not significantly influenced by variability inmesozooplankton biomass, when investigated both on inter- andintraspecific levels. Finally, when a complete seasonal cyclewas investigated at a fixed station off Plymouth (English Channel),ATCase activity was not correlated with the abundance of anymesozooplankton species apart from copepodites and adult C.helgolandicus.Furthermore, ATCase activity measured both on mesozooplanktonand female C.helgolandicus was significantly correlated (R²= 0.72, n = 33, P < 0.001) throughout the year, apart fromApril. At that particular time of the year, ATCase activitywas in phase with the peak of abundance of copepod eggs andnauplii. It is suggested that mesozooplankton peaks of ATCaseactivity reflect two periods in the life history of the copepod:embryogenesis and terminal moult. We propose further experimentsto test this hypothesis and to promote the development of molecularbiomarkers in order to characterize specific zooplankton metabolicprocesses.     

High-resolution vertical profiles of pelagic tunicates

The objectives of our research were to obtain high-resolutionvertical profiles of the abundance of pelagic tunicates andto relate their distribution to physical and biological variables.The abundance of doholids (Tunicata: Thaliacea) from near thesurface to near the seafloor was determined by obtaining continuousvideotape recording from a descending submersible. These verticalprofiles, accompanied by temperature and fluorescence measurements,revealed (i) pronounced changes in dololiolid concentrationwithin 2 m depth intervals, and (ii) two general modes of verticaldistribution, one characterized by a peak in abundance in thethennocline and the second by a more or less even distributionthroughout much of the water column. This in situ approach canprovide estimates of zooplankton abundance in near real-time     

Population dynamics of Aurelia aurita (Scyphozoa) from an isolated brackish lake, with particular reference to sexual reproduction

The annual cycle of abundance, growth and sexual reproductionof the scyphomedusa Aurelia aurita was examined over a 12 monthperiod in an isolated, shallow, brackish, man-made body of water:Horsea Lake, England. Strobilation started in December and ephyraewere present through June in varying numbers, suggesting a 7month period of near-continuous recruitment. Horsea Lake hasa dense A.aurita population, with the peak abundance of 24.9m^–3 occurring in mid-May. Medusae (>10 mm) were presentin the water column from March through January. Maximum belldiameter of 105 mm was observed in late September. However,medusa growth was very poor (G ranged between –0.03 and0.07 day^–1 over a 12 month period), and monthly mean belldiameters were typically <50 mm. For most of the year, theabundant A.aurita population was most likely severely food limitedas a result of a numerically and species-poor mesozooplanktoncommunity dominated by the calanoid copepods Acartia margalefiand Acartia tonsa. Nevertheless, the very small A.aurita medusaewere still able to attain sexual maturity and reproduce. Ripefemale medusae carrying fertilized eggs and planula larvae werefirst observed in May and June, although the main period ofplanula larva production was September-December. In November,all females were ripe, the smallest being 19–20 mm belldiameter. At both Horsea Lake and a nearby productive estuary,Southampton Water, planula larva production increased linearlywith wet weight, although compared with the larger A.auritamedusae from Southampton Water, reproductive output at HorseaLake was low. Aurelia aurita in Horsea Lake showed plasticityin its population characteristics in response to environmentalconditions, with both fecundity and size to maturity varyingon a seasonal basis. Medusae appeared to partition the availablefood resources into either somatic growth when food was abundantor reproductive effort when food was scarce.     

The abundance cycle and influence factors on red tide phenomena of Noctiluca scintillans (Dinophyceae) in Dapeng Bay, the South China Sea

The seasonal occurrence of Noctiluca scintillans and the causativemechanisms of red tide phenomena were investigated in the northwesternpart of Dapeng Bay, the South China Sea, from March 1990 toJune 1992. It occurred in the plankton usually from Januarythrough May/June with peak abundance from March to early May.At least eight red tides of this organism, when the surfacewater was covered with the pink patches and slicks, were observedduring the investigation period, and all of them took placebetween March and early May. Noctiluca occurred at temperaturesfrom 15.8 to 2.8.6°C, and the population density was thehighest at temperatures between 19 and 25°C. It disappearedfrom the plankton in summer and fall when the temperature warmedto lethal level of     

The demography of Calanus finmarchicus (Copepoda: Calanoida) in the Middle Atlantic Bight, USA, 1977-2001

The spatial distribution and annual cycle of abundance for thelate-stage copepodites of the calanoid copepod Calanus finmarchicusare described for the shelf and slope waters of the Middle AtlanticBight (MAB), a subregion of the US Northeast large marine ecosystem.Samples were collected with bongo nets from 1977 to 2001 onbroad-scale surveys of the region and from a continuous planktonrecorder (CPR) towed at 10-m depth along a route that traversedthe area. The copepod’s abundance in shelf waters increasedrapidly in early spring and reached maximum levels in May–June.It declined slowly thereafter, until a minor secondary pulseoccurred in late autumn, which was followed by the annual winterlow. Slope water abundance of C. finmarchicus peaked in Apriland was virtually absent there at 10-m depth from July throughthe following February. The two samplers, both portrayed similarhigh interannual abundance variability with no long-term trendevident for the 25-year period. Cluster analysis pinpointedthree shelf areas with similar abundance patterns, one of whichwas a region of high density located in the northeastern offshorewaters. Evidence is presented that indicates the source of thishigh abundance is likely from the populations that overwinterin deep water basins of the Gulf of Maine (GOM). The copepod’sinterannual abundance variability was found to be negativelycorrelated with water temperature and unrelated to fluctuationsof the North Atlantic oscillation (NAO) index.     

Distribution and abundance of larvae of palaemonid prawns in the ria de Vigo, N.W. Spain

The distribution of the larval stages of the members of thePalaemonid family present in the ria dc Vigo in N.W. Spain isstudied. The highest abundance of larvae of all species occursin August, the most abundant being Palaemon adspersus and P.serratus. The larvae of these species are found mainly in thetop 0.5 m of water in straits or inlets of the ria.     

The annual cycle of planktonic ciliates in nearshore waters at Signy Island, Antarctica

The abundance and biomass of marine planktonic ciliates in BorgeBay, Signy Island, were determined at monthly intervals betweenApril 1990 and June 1991. At least 24 different ciliate taxawere recorded from samples preserved in Lugol's iodine, includingthe tintinnids Codonellopsis balechi, Cymalocylis convallaria,Laackmaniella naviculaefera and Salpingella sp., and the aloricatetaxa Didinium sp. and Mesodinium rubrum. Ciliate abundance andbiomass exhibited a clear seasonal cycle with high values duringthe austral summer and low values in the austral winter. Abundanceranged from 0.3 10³l^–1 in September to 2.3 10³l^–1in January, while biomass ranged from 0.5 µg C l^–1in October to 12.6 µg C l^–1 in December. Small ciliatesdominated abundance throughout the year, and biomass duringwinter. Larger ciliates contributed most to biomass during summer.Aloricate ciliates were common throughout the year, while tintinnidscontributed substantially to abundance and biomass only duringsummer. Salpingella sp. was the commonest tintinnid, but C.convallariacontributed most to tintinnid biomass. The seasonal patternof ciliate abundance and biomass matched that of chlorophylla concentration and bacterial biomass, suggesting tight trophiccoupling between ciliates and other components of the pelagicmicrobial community. ¹Present address: Scott Polar Research Institute, Universityof Cambridge, Lensfield Road, Cambridge CB2 1ER, UK     

Relationship between the bloom of Noctiluca scintillans and environmental factors in the coastal waters of Sagami Bay, Japan

In order to study the mechanism of the bloom formation of Noctilucascintillans, the relationship between the variation in the abundanceof N. scintillans and environmental factors was examined inthe coastal waters of Sagami Bay, Japan. Hydrographic (temperature,salinity, water stability), biological (chlorophyll a concentration,zooplankton biomass) and meteorological (rainfall, wind velocity,wind direction) factors were investigated from 1997 to 2004.For all years, the abundance of N. scintillans started to increasefrom March and reached a maximum in spring between April andMay. The abundance in 1997 and 2000 was relatively high comparedto the other years while the abundance in 1998 and 2004 wasrelatively low. A stepwise multiple linear regression analysisshowed that the wind direction and rainfall were significantlycorrelated with the variation in the abundance of N. scintillans.Our results suggest that bloom formation can be separated intoa three-step process: (i) initial increase in the abundanceof N. scintillans attributed to an increase in optimum hydrographicand biological factors, (ii) N. scintillans is then accumulatedby convergence of seawater by the factors of low rainfall andwind and (iii) swarmer-effects suggested enhanced bloom formation.Accumulation is considered to be a key trigger in this processof the formation of large-scale blooms. This paper was presented at Plankton Symposium III, held atFiguera da Foz, Portugal between 17 and 20 March 2005, underthe auspices of the University of Coimbra and the Universityof Aveiro, and coordinated by Mário Jorge Pereira andUlisses M. Azeiteiro.     

Abundance, breeding and temporal size distribution of the chaetognath Sagitta setosa in the Kattegat

During a hydrographical survey in the northern Kattegat, 1975to 1977, vertical plankton hauls were taken monthly at fourstations between Göteborg and Frederikshavn. Hydrographicalparameters were measured at 10 stations along the transect.Sagitta setosa J. Mailer was by far the most dominant chaetognath(99% of individuals). Sagitta elegans Verrill and Eukrohniahamata Möbius occurred simultaneously with inflowing highsaline water during autumn and winter. The transport of S. setosaby currents was an important factor influencing the abundancesand the breeding period in the Kattegat. The abundance peakin 1977 was only one fifth of that found in 1975. The startand the duration of the breeding period (defined as presenceof stage 1 individuals <3 mm) varied between the three years.Breeding occurred between the middle of July and December witha peak in August and September. Changes in size distributionwere small during winter and spring, possibly due to low temperatures.S. setosa has probably a life span of one year in the Kattegat.Some earlier results regarding the number of breeding periodsof S. setosa per year are questioned. It is proposed that thenumber of generations per year of S. setosa and the start andthe duration of the breeding period(s) in western European watersshould be regarded as an open question until further informationis available.     

Spatio-temporal patterns in the copepod community in Malangen, Northern Norway

The objective of this study was to identify the key copepodspecies and their life cycles, and provide evidence for anyseasonal and spatial changes in the copepod community in Malangen,a fjord located 30 km to the south of Tromsø in NorthernNorway (69°30'N, 18°21'E). As a result of high levelsof freshwater run-off in May, the fjord became highly stratifiedwith a sharp pycnocline at 10–30 m depth from May to August.The generation patterns of six copepod species are described.Calanusfinmarchicus produced one generation during the spring thatyear, whereas two generations appeared to be produced by bothPseudocalanus acuspes and P.minutus: one in spring (March-June)and the other in autumn (August-December). However, it is uncertainto what extent P.minutus regularly produces a second generation.Two peaks of CI-CIII Metridia spp. were found; there were differencesalong the length of the fjord in the timing of these, but therelative contributions of M.longa and M.lucens are uncertain.Chiridius armatus CI-CIII peaked in abundance in the spring,which indicates that one main generation was produced at theouter station of the fjord. The copepod community in Malangencould be grouped into three entities according to their numericalabundance during the year one group of highly abundant forms,generally with maxima >50 000 individuals m^–3 (C.finmarchicus,Microcalanus sp., Oithona similis , Oithona spinirostris, Acartiasp. and Pseudocalanus spp.), a second group of less abundantspecies with a clear seasonality in abundance, varying from500 to 50 000 individuals m^–3 (M.longa, M.lucens, Calanushyperboreus, Carmatus, Tenwra longicornis, Oncaea sp., Euchaetanorvegica and Scolecithrwella minor), and a third group of 14holoplanktonic species, sporadically occurring in the fjord.The study demonstrates clear gradients in the abundance of fivespecies along the length of the fjord: the recruiting generationof C.finmarchicus occurred in higher abundances at the outerstation in May and June compared to the other inner sites. Laterin the season, the reverse situation appeared, in which thepopulation was more abundant in the inner part of the fjorcCalanw hyperboreus increased abruptly in abundance from lowwinter levels to a maximum in April-May, and declined steadilyduring the season (except at the innermost station). Metridialucens, M.longa and C.armatus demonstrated different distributionpatterns in Malangen that matched their preferred areas of distribution.Both M.lucens and C.armatus are known as oceanic and deep-waterspecies, respectively, and these were prevalent at the two outersites in Malangen. Metridia longa is a more nentic species andwas found in highest numbers at the two innermost sites. Themechanisms for the differences in abundance among these specieswithin the fjord are discussed.     

Behavioral and Morphological Adaptations by Galapagos Land Iguanas (Conolophus subcristatus) to Water and Energy Requirements of Eggs and Neonates

We examine the importance of both the hydric environment ofnaturally incubating reptilian eggs and the energetic needsof hatchlings via an investigation of reproduction in Galapagosland iguanas (Conolophus subcristatus). Hatching success ofegg clutches and the size of subsequent hatchlings are bothpositively correlated with the water potential of natural nests,as predicted from previous laboratory experiments. Water potentialsrepresenting optimal incubation environments are available foronly a brief period in nature, and depend upon the same seasonalrainfall as does food abundance for the emerging hatchlings.The temporal placement of the reproductive season of Conolophussubcristatus balances these conflicting needs for water by eggsand energy by hatchlings. Oviposition occurs slightly beforesuitable water potentials are reached and hatchling emergenceoccurs after the peak in food abundance. Morphological adaptations by Conolophus subcristatus to theirprecarious reproductive phenology include greater amounts ofalbumen in their eggs, and greater energy reserves in emergenthatchlings than most other lizard species. These adaptationslessen the severity of an arid environment where water becomesavailable for periods too short to allow both oviposition andhatching to be temporally placed in an optimal manner.     

Ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA, during 1972-1977

Plankton dynamics at a station in lower Narragansett Bay, RIare compared for six summer and fall seasons, 1972–1977.In four of these years, initiation of the summer pulse of thectenophore Mnemiopsis leidyi was accompanied by a rapid declinein zooplankton abundance and a summer phytoplankton bloom. Terminationof the phytoplankton bloom coincided with depleted ctenophoreabundance and increased zooplankton biomass in two of the years.Yearly variations in the summer abundance of the diatom Skeletonemacostatum were positively related to the magnitude of the ctenophorepulse. The magnitude of ctenophore population was related tothe zooplankton biomass present at the start of the pulse. Theserelationships, the timing and magnitude of the plankton eventssuggest that M. leidyi regulated summer zooplankton and phytoplanktondynamics. Ctenophores may control phytoplankton blooms indirectlythrough their predation on herbivorous zooplankton and directlyby the nutrient excretion accompanying such grazing. This evidencethat a planktonic carnivore two trophic steps removed from thephytoplankton regulates the latter's dynamics in NarragansettBay is analogous to reported regulation of benthic algal (kelp)dynamics by the sea otter, lobster and various crabs throughtheir predation on herbivorous sea urchins. The factors responsiblefor the seasonal decrease in ctenophores remain unresolved;ctenophore predators on Mnemiopsis are absent in NarragansettBay. Infection by the vermiform larval anemone, Edwardsia lineata,grazing by the butterfish, Peprilus triacanthus, and changesin food availability, temperature and salinity likewise do notexplain this disappearance.     

Adult, not juvenile mortality as a major reason for the midsummer decline of a Daphnia population

We analysed dynamics and structure of a Daphnia galeata populationprior to and during a midsummer decline of this species in Bautzenreservoir (Saxony, Germany). Patterns of juvenile and adultmortality were determined by combining field data with laboratoryestimates of juvenile growth. After establishing high densities,fecundity and recruitment of D.galeata markedly decreased, whereassize at maturity was high. Immediately before the populationdecline, adult mortality increased and remained high even afterthe decline, whereas juvenile mortality was low during the wholeinvestigation period, and virtually absent after the decline.We conclude that the succession of events leading to a midsummerdecline of Daphnia is as follows. (i) A quick increase in Daphniaabundance leads to the formation of a strong ‘peak cohort’of about the same age. (ii) During the clear-water phase, foodconditions deteriorate, fecundity declines and hence, recruitmentis low. Juvenile mortality during this period is low, but present.(iii) Adult mortality increases when the ‘peak-cohort’reaches its mean life-span, which is reduced due to interactionsbetween age-specific and starvation-induced mortality. At thispoint, Daphnia population dynamics can no longer be explainedwithout the onset of size-selective predation. Hence, the timingbetween enhanced mortality due to senescence on the one handand predation on the other hand, both directed towards adultdaphnids, may be decisive for the initiation of a midsummerdecline of Daphnia.