Distribution and abundance of marine nematodes on the kelp Macrocystic integrifolia

The species and distribution of nematodes on the kelp Macrocystic integrifolia Bory, in the Bamfield region of Barkley Sound, British Columbia, Canada were examined. Nine species (belonging to six families) of nematodes were found on the kelp blades. Three species (Monhystera disjuncta, M. refringens and Prochromadorella neapolitana) comprised 91–99% of the nematode fauna and occurred in all monthly samples from July 1978 to November 1979. The three dominant species exhibited seasonal density differences. P. neapolitana occurred mostly in the summer, M. refringens abundance peaked from July to October and M. disjuncta was relatively abundant throughout the year. The other species contributed little to the overall abundance and distribution patterns. All three species occurred in greatest abundance on the lower and middle blades of M. integrifolia in the deep end of the kelp bed. Very few individuals occurred on the upper blades. Nematode distribution on M. integrifolia appeared to be related to blade age and the associated food sources on the blade.     

Impact of osmolytes on buoyancy of marine phytoplankton

Marine phytoplanktonic cells can achieve neutral buoyancy only if the excess density of their relatively heavy structural materials (proteins, carbohydrates, silicate) is compensated for by the incorporation of materials that have densities less than seawater. We have calculated densities and osmotic concentrations for several marine algae, based on published values of structural materials and concentrations of inorganic ions and other osmolytes. The calculations, incorporating the partial molal volume, molecular mass, concentrations and osmotic coefficients, indicate that most published listings of intracellular osmolytes in marine algae are insufficient to provide the turgor known to exist. Similarly, the density of phytoplanktonic cells, calculated on the basis of known or estimated concentrations of cellular components, generally exceeds the density of seawater, which would cause negative buoyancy (sinking) throughout. We use models of osmotic concentration and cellular density in which we supplement known concentrations of osmolytes with proxy osmolytes. In particular, concentrations of some 100 mol m^-3 of quaternary ammonium derivatives can explain the deficits of both osmotic concentration and buoyancy.     

BDE-47对4种海洋微藻抗氧化酶活性的影响

运用实验生态学和生物化学的方法,研究了不同质量浓度的2,2',4,4'-四溴联苯醚(BDE-47)对海水小球藻Chlorellaautotropica,牟氏角毛藻Chaetoceros muelleri,中肋骨条藻Skeletonema costatum和赤潮异弯藻Heterosigam akashiwo的抗氧化酶活性的影响.结果表明:在实验设定质量浓度范围内(0.1～2.5 μg·L~(-1)),4种微藻的超氧化物歧化酶(SOD)和过氧化氢酶(CAT)均能出现一定的应激活性,以减少BDE-47胁迫对藻细胞自身的危害,但是酶活增加的幅度却有很大不同.与赤潮异弯藻和中肋骨条藻相比,海水小球藻和牟氏角毛藻的SOD和CAT对BDE-47具有较高的敏感性.4种海洋微藻的SOD和CAT对BDE-47的敏感性顺序依次为:海水小球藻>牟氏角毛藻>中肋骨条藻>赤潮异弯藻.因此,海水小球藻的SOD和CAT可以作为指示海洋环境中BDE-47污染水平的生物标志物.     

Local disturbance cycles and the maintenance of heterogeneity across scales in marine metapopulations

In marine systems, the occurrence and implications of disturbance-recovery cycles have been revealed at the landscape level, but only in demographically open or closed systems where landscape-level dynamics are assumed to have no feedback effect on regional dynamics. We present a mussel metapopulation model to elucidate the role of landscape-level disturbance cycles for regional response of mussel populations to onshore productivity and larval transport. Landscape dynamics are generated through spatially explicit rules, and each landscape is connected to its neighbor through unidirectional larval dispersal. The role of landscape disturbance cycles in the regional system behavior is elucidated (1) in demographically open vs. demographically coupled systems, in relation to (2) onshore reproductive output and (3) the temporal scale of landscape disturbance dynamics. By controlling for spatial structure at the landscape and metapopulation levels, we first demonstrate the interaction between landscape and oceanographic connectivity. The temporal scale of disturbance cycles, as controlled by mussel colonization rate, plays a critical role in the regional behavior of the system. Indeed, fast disturbance cycles are responsible for regional synchrony in relation to onshore reproductive output. Slow disturbance cycles, however, lead to increased robustness to changes in productivity and to demographic coupling. These testable predictions indicate that the occurrence and temporal scale of local disturbance-recovery dynamics can drive large-scale variability in demographically open systems, and the response of metapopulations to changes in nearshore productivity.     

Detection of pollution effects on marine macrobenthos: further evaluation of the species abundance/biomass method

Further empirical data are presented to evaluate the method of detecting pollution-induced disturbance in marine benthic communities by a comparison of the distribution of numbers of individuals among species with the distribution of biomass among species. A suggested abbreviated name for the technique is the ABC method (abundance biomass comparison). Application of the technique to new data shows that it is a sensitive indicator of natural physical and biological disturbance as well as pollution-induced disturbance over both spatial and temporal scales. Changes in the configuration of ABC plots during ecological succession are the reverse of those resulting from increased pollution levels. The technique should also be applicable to intertidal sediments, where physical disturbance of the sediment by waves does not appear to preclude its utility.     

Seasonal variation in the diversity and abundance of pelagic larvae of Antarctic marine invertebrates

Most marine benthic macroinvertebrate species reproduce via a larval phase but attempts to explain the occurrence of different larval strategies (feeding or non-feeding, pelagic or benthic) in different habitats have been largely inconclusive. There have been very few year-round surveys of meroplankton at any latitude and in consequence fundamental data on the diversity, abundance, and timings of larval life history phases are lacking. There has been considerable debate regarding the viability of pelagic larvae in cold waters with highly seasonal primary production but there has been only one year-round study of meroplankton in the Southern Ocean, and that was outside of the Antarctic Circle. We present data from the first year-round survey of meroplankton assemblages at a location within the Antarctic Circle. We surveyed abundances of meroplanktonic larvae over 1.5 year at Rothera Point, West Antarctic Peninsula (67°34′S, 68°07′W). Larvae were collected in monthly diver-towed net samples close to the seabed at 20 and 6 m total water depths at each of three locations and were identified and counted live immediately after sampling. A total of 99 operationally defined taxonomic types representing 11 phyla were recorded but this is likely to be an underestimate of true diversity because of inherent difficulties of identification. Larvae were present in all months of the year and although planktotrophic larvae were more abundant in summer, both feeding and non-feeding types were present in all months. Comparisons of seasonal larval abundances with data from a settlement study at the same sites and from the literature show that larvae of mobile adults settle in summer regardless of developmental type, whereas sessile taxa settle in all seasons. We suggest that this is a consequence of differences in the food requirements of mobile and sessile fauna and that the availability of food for post-larval juveniles is more critical for survival than factors affecting the larval stage itself.     

Impact of sewage on the distribution,abundance and community structure of rocky intertidal macro-organisms

The biological effects of a low-volume domestic sewage discharge were studied near Wilson Cove, San Clemente Island, California (USA), from February to June, 1972. There were fewer species and less cover near the outfall (7 macro-invertebrates, 17.6%; 13 macrophytes, 91.7%) than in nearby “unpolluted” control areas (9 macroinvertebrates, 9.2%; 30 macrophytes, 103.4%). The outfall biota was less diverse than that of the controls, as shown by 5 different diversity indices. A great reduction in community stratification (spatial heterogeneity) and, hence, community complexity occurred near the outfall; this reduction in stratification was primarilly due to the absence of Egregia laevigata, Halidrys dioica, Sargassum agardhianum and Phyllospadix torreyi. These were replaced in the mid-intertidal near the outfall by a low turf of blue-green algae, Ulva californica, Gelidium pusillum and small Pterocladia capillacea, and in the lower intertidal by Serpulorbis squamigerus covered with Corallina officinalis Var. chilensis. A statistically-based determination of assemblages or groups of organisms (i.e., cluster analysis) revealed 3 discrete outfall and 3 discrete control area groups; 3 assemblages contained samples from both areas. The distributional patterns of these groups indicate that near the outfall the degree of dilution of discharged sewage is more important in regulating zonation than is tidal height. The enhancement of the suspension feeder Serpulorbis squamigerus and the omnivores Ligia occidentalis, Pachygrapsus crassipes and Anthopleura elegantissima in the outer fringe of the outfall plume hypothetically is due to their ability to utilize sewage as a food source. A critical effect of the outfall may be to decrease environmental stability thereby favoring rapid-colonizers and more sewage-tolerant organisms. The outfall macrophytes were characterized by relatively higher net primary productivities, smaller growth forms, simpler and shorter life histories, and most were components of early successional stages.     

Effects of physical disturbance on an estuarine intertidal community: field and mesocosm results compared

The response of an estuarine macrobenthic community to different intensities of physical disturbance was examined using comparable field and mesocosm experiments. In the field study, areas of sediment (2500 cm²) were disturbed by surface-raking at a range of disturbance intensities: no disturbance, once every 2 mo, once every month, once every 2 wk, once or three times every week. These disturbance regimes were also imposed on buckets of sediment (855 cm²) collected from the field study site and maintained within a mesocosm system. In the univariate data analysis, neither field nor mesocosm communities displayed any trends in relation to the frequency of disturbance. Diversity indices were generally reduced in the mesocosm communities compared with the field communities. Multivariate analysis indicated significant differences between the field communities subjected to the most intense disturbance and those communities disturbed every ±2 wk or less. These differences were attributed to reductions in the five numerically dominant species in the high- disturbance treatments. More statistically significant differences were observed between the different disturbance intensities in the mesocosm experiment than in the field study. The implications of differences between field and mesocosm experiments results are discussed. Received: 14 June 1999 / Accepted: 3 December 1999     

Photosynthetic characteristics of marine phytoplankton from contrasting physical environments

Two suites of phytoplankton samples have been collected in consecutive years at various times over a day from selected depths within vertically mixed and stratified water columns in the western Irish Sea, in order to provide a range of possible light histories within the populations collected. Values for the maximum rate of ¹⁴C retention (P _max) and the initial slope of the ¹⁴C retention: light intensity curve () were obtained. Supra-thermocline samples from the stratified water exhibited higher P _max values than corresponding subthermocline samples. Higher values of were also generally associated with samples from the supra-thermocline zone of the stratified region. Differences in the depth distribution of P _max and in the mixed water were small, except in the presence of a shallow thermocline. In one suite of samples from the stratified water, a diurnal increase in the P _max values of the supra-thermocline samples was observed. P _max values obtained from the samples from the mixed water were interpreted in relation to the distribution obtained from the samples from the stratified zone. Data from both the contrasting sites visited for one sample suite demonstrated a two-phase relationship between the chlorophyll a concentration and both P _max and . The rates of ¹⁴C retention of the first suite of samples were estimated by two techniques. The average differences in the retention were greater in samples from the sub-as opposed to suprathermocline zone. No trends were apparent in the smaples from the mixed waters.     

Effects of long-term physical disturbance by commercial scallop fishing on subtidal epifaunal assemblages and habitats

 This paper examines spatial differences in the distribution of by-catch assemblages from the scallop [Pecten maximus (L.) and Aequipecten opercularis (L.)] fishing grounds in the North Irish Sea, during 1995. The sites examined have been exposed to differing known levels of fishing disturbance by scallop dredging, based on unusually high-resolution data extracted from fishermens' logbooks. Uni- and multi-variate techniques have been used on a production dataset (a value which incorporates both abundance and biomass figures), as well as abundance and biomass data individually. The original species list was reduced to higher taxonomic groupings in line with the theory that the latter is more appropriate for detecting anthropogenic change. Species diversity and richness, total number of species, and total number of individuals all decrease significantly with increasing fishing effort. Species dominance increases with effort. Total abundance, biomass and production, and the production of most of the major individual taxa investigated decrease significantly with increasing effort. Multivariate analysis reveals a significant relationship between fishing effort and by-catch assemblage structure. The taxa most responsible for the differences are the echinoids and cnidarians, but prosobranch molluscs and crustaceans also contribute to the differences. By-catch assemblage structure is more closely related to fishing effort than any other environmental parameter investigated, including depth and sediment type. We observed an approximately linear decrease in diversity with increasing fishing disturbance, and suggest this is primarily due to selective removal of sensitive species and, more importantly, habitat homogenisation. These results were interpreted in the light of ecological theories relating disturbance to community structure. The argument that invertebrate scavenger populations benefit from prolonged exposure to fishing disturbance was also examined, but no supporting evidence was found. Received: 29 January 2000 / Accepted: 31 May 2000     

Effects of physical disturbance on nematode communities in sand and mud: a microcosm experiment

A microcosm experiment was carried out to evaluate the effects of continuous and spasmodic physical disturbance of differing frequency on the structure of nematode communities of intertidal sand and mud. There was a marked, characteristic change in abundance and diversity for both sediment types. In the sand microcosms, the majority of univariate measures of community structure, including species diversity, were lowest in the sediments subjected to a high frequency of disturbance. For the mud microcosms, most univariate measures reached their highest values in the treatments with an intermediate frequency of disturbance and were lower in treatments subjected to both higher and lower frequencies. Multivariate ordinations for both nematode assemblages showed a clear separation of undisturbed controls and disturbed treatments, but only for the muddy sediment was there a graded change in community composition with increasing frequency of disturbance. These results confirmed our a priori expectation that nematode assemblages from mobile sandy sediments would be more resilient to physical disturbance than those from sheltered muds, and these observations are considered in the context of Connell's intermediate disturbance hypothesis. Received: 7 May 1997 / Accepted: 10 October 1997     

Effects of environmental parameters on the oxygen consumption of four marine invertebrates: a comparative factorial study

The oxygen consumption curves of two decapod crustaceans (Palaemon serratus, Penaeus monodon) and two prosobranch molluscs (Trunculariopsis trunculus, Nassarius mutabilis) have been detected in the entire pO₂ interval from 0 to 160 mmHg, under different conditions of temperature and salinity. From the experimental curves, physiological parameters such as the initial oxygen consumption velocity, the Q ₁₀ values and the oxygen independence indices have been measured. The latter parameters have been obtained using normalised plots which allow their better evaluation. The effects of temperature, salinity and oxygen partial pressure on the oxygen-consumption features have been studied using a factorial experimental plan which allows measurement of the effects of each experimental variable as well as the effects of synergistic interactions between different variables. Received: 27 March 2000 / Accepted: 13 November 2000     

Patterns of micro- and meiofaunal abundance in marine sediments,measured with the adenosine triphosphate assay

Adenosine triphosphate (ATP) measurements are used to determine vertical and seasonal distributions of microorganisms and meiofauna in sediments from a 14 m-deep mud bottom in central Long Island Sound on 12 sampling dates from April 1975 to October 1976. Below the topmost 1 cm of sediment, ATP measurements can be useful in estimating and comparing standing stocks of microorganisms and meiofauna. In the top 1 cm, however, large quantities of newly settled bivalves (Yoldia limatula, Nucula annulata, and Mulinia lateralis) and juvenile polychaetes (Owenia fusiformis) in summer and fall months account for total ATP concentrations. The ATP content of individual meiofauna ranges from 1.97 ng individual copepod nauplius^-1 to 190.7±60 ng individual M. lateralis ^-1. In general, the total ATP content of individual polychaetes and bivalves is much higher than that of individuals of other groups. However, on a mg ATP per g wet or dry tissue basis, the ATP content of micro- and meiofaunal taxa are not significantly different. In addition to providing a means for comparing micro- and meiofaunal standing stocks, ATP measurements permit examination of the relative contribution of different meiofauna to the total living biomass of meiofauna in sediments. Total sediment ATP concentrations are greatest in the top 1 cm at all seasons, and decrease with increasing depth in the sediment. Annual concentrations in the topmost centimeter average 4.22 g g dry sediment^-1 and range seasonally from 1.09 to 7.64 g g dry sediment^-1. At a depth of 10 cm, values average 0.16 and range from 0.019 to 0.35 g g dry sediment^-1. High ATP concentrations in surface sediment reflect high concentrations of microorganisms and meiofauna at the sediment-water interface. The top 2 cm of sediment contain 71% of all meiofauna, with 41% occurring in the topmost cm. In general, densities are lowest in the winter and highest during the spring and summer, averaging 490 individuals 10 cm^-2, and varying from 87 to 1366 individuals 10 cm^-2. Because of wide variation in recruitment patterns of the benthos in Long Island Sound, the extremes of the range in meiofaunal densities can be observed in the same month in two different years. In order to monitor and compare standing stocks of organisms less than 1 mm in size in sediments, the ATP assay can save hours of processing time compared with alternate methods such as direct counts.     

Impact of ocean acidification on marine ecosystems: educational challenges and innovations

Population growth and social/technological developments have resulted in the buildup of carbon dioxide (CO₂) in the atmosphere and oceans to the extent that we now see changes in the earth’s climate and ocean chemistry. Ocean acidification is one consequence of these changes, and it is known with certainty that it will continue to increase as we emit more CO₂ into the atmosphere. Ocean acidification is a global issue likely to impact marine organisms, food webs and ecosystems and to be most severely experienced by the people who depend on the goods and services the ocean provides at regional and local levels. However, research is in its infancy and the available data on biological impacts are complex (e.g., species-specific response). Educating future generations on the certainties and uncertainties of the emerging science of ocean acidification and its complex consequences for marine species and ecosystems can provide insights that will help assessing the need to mitigate and/or adapt to future global change. This article aims to present different educational approaches, the different material available and highlight the future challenges of ocean acidification education for both educators and marine biologists.     

Impact of aquacultures on the marine ecosystem: Modelling benthic carbon loading over variable depth

In order to be used within Environmental Impact Assessment study, we have developed a three-dimensional particle tracking model for prediction of benthic carbon loading (BCL) caused by fish farms. The model is based on stochastic differential equations for particle transport consistent with the well-known semi-empirical advection/diffusion equation. It requires only easily obtainable input data in the form of measured current record, the source location and a specification of local bathymetry. The model accounts for advection by long-term residual and tidal currents, turbulent diffusion, realistic bathymetry and variations in daily (monthly or yearly) emissions from fish farm.Here, we concentrate on the changes in sedimentation pattern caused by various bathymetric shapes. Examination of idealized cases reveals where and why we can expect the worst impact on benthic communities. For future reference, these results will be included into guidelines for fish farming.     

Impact of conditions of cultivation and adsorption on antimicrobial activity of marine bacteria

The potency of free-living and animal-associated marine bacteria to produce antimicrobial substances has been studied in 491 strains isolated from northern and southern parts of the Pacific Ocean. A total of 26% (126 out of 491) of the strains examined produced antimicrobial compounds against 11 test bacterial strains (TBS) including the fish pathogens Aeromonas hydrophila and Vibrio anquillarum. Antimicrobial substances (AS) produced by marine bacteria were especially active against Staphylococcus epidermidis, Proteus vulgaris, Enterococcus faecalis, and Candida albicans. Twelve strains, isolated from different sources, were chosen as promising candidates, producing a number of AS. Production of AS varied within 24 to 72 h, increasing in a culture medium based on natural sea water with Br-ions, and after attachment to polymeric surfaces. In order to study the influence of adsorption, selected strains with a high potential for antimicrobial production were cultivated on polymeric surfaces with different hydrophobicities and chemical functionalities. These parameters of the surface hydrophobicity (measured by means of water contact angles) and chemical functionality of the surfaces were manipulated using the photo- and thermochemistry of a polymeric system (diazo-naphto-quinone/novolak) commonly used as a photoresistant material in semiconducto-manufacturing. The highest antimicrobial activities occurred on hydrophilic surfaces (standard exposed photoresistant films), whereas the number of attached cells was greater on hydrophobic surfaces, characterized as unexposed resistant films. These results suggest that the chemical nature of induced hydrophilicity may also be a major factor in controlling antimicrobial activity of adsorbed bacteria. Received: 5 March 1997 / Accepted: 24 August 1997     

Food ingestion by the marine planktonic copepod Paracalanus in relation to abundance and size distribution of food

Juveniles and adult females were presented a food spectrum of three algae of different sizes (4.5, 12 and 20 m cell width). The increase in rate of ingestion of the medium-sized alga with an increase in copepod size was significantly greater than the increase in rate of ingestion of the small alga. It is hypothesized that the perception of chemical signals from the small alga by a copepod decreases as the copepod moults from stage to stage. The rate of ingestion of the large alga by copepod stage V (CV) and adult females was lower than the rate of ingestion of the medium-sized alga at mid- and high phytoplankton concentrations. The amount of nitrogen ingested when the medium-sized alga alone was offered was either higher than (stage C II) or not significantly different from that when the three algae were offered together (stage C IV). Ingestion rates are reduced when there is a multialgal food source. This implies that there is increased stability in the ocean because multiparticle food sources are more slowly depleted than unialgal foods. Weight-specific ingestion rates of copepods fed the three algae simultaneously increased from nauplius to stage C III and then decreased as adulthood approached. The contribution of the small alga to the total amount of nitrogen ingesied was greatest for naupliar stages while the contribution of the medium-sized cells was greater for later stages. The largest alga was readily ingested by stage C V and adult females but never contributed more than 25% of the nitrogen ingested. Eight to 12% of the nitrogen ingested by adult females was from the small alga. It is hypothesized that the algal cell size for maximum nitrogen ingestion in upwelled waters is relatively small, round or square and close to the size threshold below which adult females do not sense individual cells.     

Comparative species composition and relative abundance of decapod crustaceans in marine habitats of Panamá

The community structure of the decapod crustacean fauna of 7 tropical, shallowwater, marine habitats (sandy beaches, mangrove swamps and rocky intertidal habitats on both the Pacific and Caribbean coasts of Panama, and Pocillopora damicornis coral habitat of the Bay of Panama) were examined and analyzed for species composition and relative abundances. Collections from the 7 habitats yielded 4361 individuals, representing 236 species. The number of species per habitat was (Pacific, Caribbean): sandy beach (16, 7); mangrove (20, 17); P. damicornis (53); rocky intertidal (78, 67). There were more species represented by more individuals in the Pacific habitats. An index of faunal similarity was calculated for each pair (Pacific-Caribbean) of habitats. This index is the number of ecologically similar congeneric species which occurred in both habitats expressed as a percentage of the total number of species present in the pair of habitats. For the sandy beach communities there were three Pacific species which were similar to three Caribbean species, a similarity of 6/23 or 26%. The index of similarity for the mangrove communities is 54% and for the rocky intertidal communities it is 37%. The P. damicornis community has affinities with the Pacific rocky intertidal community (18%), with that of the Caribbean rocky intertidal (16%) and with that of Indo-West Pacific pocilloporid corals (20%). A few specialized species dominated each of the communities. The habitats and the number of species accounting for over half of the individuals present are (Pacific, Caribbean): sandy beach (1, 1); mangrove (4, 4); P. damicornis (5); and rocky intertidal (3, 6). Most of the species in each community were represented by one or a few individuals.     

Density dependence and density independence during the early life stages of four marine fish stocks

Recruitment variability caused by density-dependent and density-independent processes is an important area within the study of fish dynamics. These processes can exhibit nonlinearities and nonadditive properties that may have profound dynamic effects. We investigate the importance of population density (i.e., density dependence) and environmental forcing (i.e., density independence) on the age-0 and age-1 abundance of capelin (Mallotus villosus), northeast Arctic cod (Gadus morhua), northeast Arctic haddock (Melanogrammus aeglefinus), and Norwegian spring spawning herring (Clupea harengus) in the Barents Sea. We use statistical methods that explicitly account for nonlinearities and nonadditive interactions between internal and external variables in the abundance of these two pre-recruitment stages. Our results indicate that, during their first five months of life, cod, haddock, and herring experience higher density-dependent survival than capelin. The abundance of age-0 cod depends on the mean age and biomass of the spawning stock, a result which has implications for the management of the entire cod stock. Temperature is another important factor influencing the abundance at age-0 and age-1 of all four species, except herring at age-1. Between age-0 and age-1, there is an attenuation of density-dependent survival for cod and herring, while haddock and capelin experience density dependence at high and low temperatures, respectively. Predation by subadult cod is important for both capelin and cod at age-1. We found strong indications for interactions among the studied species, pointing to the importance of viewing the problem of species recruitment variability as a community, rather than as a population phenomenon.     

Seasonal fluctuations in the abundance of populations of marine cladocerans and their resting eggs in the Inland Sea of Japan

Seasonal fluctuations in the abundance of resting eggs of four species of marine cladocerans, Penilia avirostris Dana, Podon polyphemoides (Leuckart), Evadne tergestina Claus and Evadne nordmanni Lovén, were investigated in association with their planktonic populations in the central part of the Inland Sea of Japan for a period of over two years. Most numerous were the eggs of P. avirostris, followed by those of the species in the aforementioned order. There was a marked seasonal fluctuation in the number of eggs that was closely correlated with the occurrence of the planktonic population for each species. The egg-number was highest just before the planktonic population disappeared from the water column, and gradually decreased thereafter until a minimum value was reached shortly before the new planktonic population appeared in the next season. The pattern of fluctuation in number was found to be very similar among all four species, indicating strongly that the cladoceran populations in this warm temperate sea may persist either as plankton in warmer seasons or as benthic resting eggs in colder seasons. The possible causes of the decrease in egg density in bottom sediments are discussed.