Replacement of large copepods by small ones with eutrophication of embayments: cause and consequence

Species and size compositions of the planktonic copepod community were compared between two eutrophic embayments, Tokyo Bay and Osaka Bay. Within these bays, the median body weight of the community was lowest at the innermost station and increased gradually offshore. In Tokyo Bay, which is more eutrophic than Osaka Bay, the dominant species was Oithona davisae and the median carbon weight of the community was ca 0.1 µg. In Osaka Bay, Paracalanus sp., Calanus sinicus and Corycaeus spp. were dominant and the median weight was 1–2 order of magnitude higher (2–78 µg) than in Tokyo Bay. Some 40 years ago, when eutrophication was less prominent, Acartia omorii, Paracalanus sp. and Microsetella norvegica comprised a considerable portion of the community in Tokyo Bay. The proliferation of O. davisae might have been caused with the recent change in food particle composition and/or dissociated life cycle of this species from the benthic anoxic layer. Decrease in the size composition of the copepod community may make the embayment ecosystem unfavorable for planktivorous fish, but not for jellyfish, e.g. Aurelia and Bolinopsis. This may also result in an acceleration of the flux of biogenic materials from the embayment to the outer ocean.     

Numerical increases and distributional shifts of Chrysaora quinquecirrha (Desor) and Aurelia aurita (Linné) (Cnidaria: Scyphozoa) in the northern Gulf of Mexico

Fisheries resource trawl survey data from the National Marine Fisheries Service from a 11–13-year period to 1997 were examined to quantify numerical and distributional changes of two species of northern Gulf of Mexico scyphomedusae: the Atlantic sea nettle, Chrysaora quinquecirrha (Desor), and the moon jelly, Aurelia aurita (Linné). Trawl surveys were grouped into 10 statistical regions from Mobile Bay, Alabama to the southern extent of Texas, and extended seaward to the shelf break. Records of summertime C. quinquecirrha medusa populations show both an overall numerical increase and a distributional expansion away from shore in the down-stream productivity field of two major river system outflows: Mobile Bay and the Mississippi-Atchafalaya Rivers. In addition, there is a significant overlap between summer C. quinquecirrha and lower water column hypoxia on the Louisiana shelf. In trawl surveys from the fall, A. aurita medusae showed significant trends of numerical increase in over half of the regions analyzed. For both species, there were statistical regions of no significant change, but there were no regions that showed significant decrease in number or distribution. The relationships between natural and human-induced (e.g. coastal eutrophication, fishing activity and hard substrate supplementation) ecosystem modifications are very complex in the Gulf of Mexico, and the potential impact of increased jellyfish populations in one of North America's most valuable fishing grounds is a most critical issue. Several hypotheses are developed and discussed to guide future research efforts in the Gulf of Mexico.     

Early life history of a seahorse, Hippocampus mohnikei, in Tokyo Bay, Japan

The early morphological development, seasonal and spatial occurrence patterns, and food habits of a seahorse, Hippocampus mohnikei, in offshore waters of Tokyo Bay, central Japan, were studied on the basis of 206 juvenile and young specimens (6.0–65.3 mm TL) collected between August 1995 and January 1999. All the specimens were collected within the period from May to January, inclusive, each year. In the least developed specimen (6.0 mm TL), the number of dorsal, anal, and pectoral fin rays had attained the adult complement, whereas the minute caudal fin, consisting of two rays, was present in juveniles of 6.0–26.4 mm TL. Hippocampus mohnikei≧35 mm TL, being larger than settlement size (ca. 30 mm TL), had very low gut fullness index values (GFI = 0, >70% of specimens), whereas those of 15–29 mm TL had higher values (GFI = 2–4, >80% of specimens). In addition, larger individuals selectively fed on larger planktonic animals (species of Brachyura), which occurred naturally at low densities, although smaller food items, such as Oithona davisae and Penilia avirostris, occurred abundantly, being consumed by smaller H. mohnikei individuals (15–34 mm TL). These results indicated that food availability for H. mohnikei in offshore waters of Tokyo Bay is significant for individuals larger than settlement size, because their food preference would shift from smaller food items to larger food items, which would be scarce in their environments. Received: January 12, 2001 / Revised: May 13, 2001 / Accepted: June 14, 2001     

In situ estimation of ephyrae liberated from polyps of Aurelia aurita using settling plates in Tokyo Bay, Japan

The continuous changes in the number of newly established polyps of Aurelia aurita (L.) on settling plates under natural conditions were observed from August 1998 to September 1999 in Tokyo Bay, Japan. A sharp decline in survivorship of newly settled polyps was observed within the first few days, however, survivorship of polyps settled in October increased by budding up to 399% after two months. The number of discs in each strobila varied from 1 to 6, however, most of the strobilae formed single discs. The percentage ratios of the total number of ephyrae to the initial number of polyps on settling plates were generally lower than 10%, but the highest ratio of 594.4% was estimated for the polyps settled in October. It is considered that most of the liberated ephyrae originate from the polyps settled in October in Tokyo Bay. This study suggests that the occurrence of ripe medusae with planula larvae throughout the year contributes to the success of settlement and growth of the polyp stage in Tokyo Bay.     

Seasonal cycles of zooplankton from San Francisco Bay

The two estuarine systems composing San Francisco Bay have distinct zooplankton communities and seasonal population dynamics. In the South Bay, a shallow lagoon-type estuary, the copepods Acartia spp. and Oithona davisae dominate. As in estuaries along the northeast coast of the U.S., there is a seasonal succession involving the replacement of a cold-season Acartia species (A. clausi s.l.) by a warm-season species (A. californiensis), presumably resulting from the differential production and hatching of dormant eggs. Oithona davisae is most abundant during the fall. Copepods of northern San Francisco Bay, a partially-mixed estuary of the Sacramento-San Joaquin Rivers, organize into discrete populations according to salinity distribution: Sinocalanus doerrii (a recently introduced species) at the riverine boundary, Eurytemora affinis in the oligohaline mixing zone, Acartia spp. in polyhaline waters (18–30\%), and neritic species (e.g., Paracalanus parvus) at the seaward boundary. Sinocalanus doerrii and E. affinis are present year-round. Acartia clausi s.l. is present almost year-round in the northern reach, and A. californiensis occurs only briefly there in summer-fall. The difference in succession of Acartia species between the two regions of San Francisco Bay may reflect differences in the seasonal temperature cycle (the South Bay warms earlier), and the perennial transport of A. clausi s.l. into the northern reach from the seaward boundary by nontidal advection.Large numbers (>10⁶ m^–3) of net microzooplankton (>64 µm), in cluding the rotifer Synchaeta sp. and three species of tintinnid ciliates, occur in the South Bay and in the seaward northern reach where salinity exceeds about 5–10 Maximum densities of these microzooplankton are associated with high concentrations of chlorophyll. Meroplankton (of gastropods, bivalves, barnacles, and polychaetes) constitute a large fraction of zooplankton biomass in the South Bay during winter-spring and in the northern reach during summer-fall.Seasonal cycles of zooplankton abundance appear to be constant among years (1978–1981) and are similar in the deep (>10 m) channels and lateral shoals (<3 m). The seasonal zooplankton community dynamics are discussed in relation to: (1) river discharge which alters salinity distribution and residence time of plankton; (2) temperature which induces production and hatching of dormant copepod eggs; (3) coastal hydrography which brings neritic copepods of different zoogeographic affinities into the bay; and (4) seasonal cycles of phytoplankton.     

Spatiotemporal occurrence and feeding habits of tonguefish,Cynoglossus lighti Norman, 1925, larvae in Ariake Bay,Japan

Spatiotemporal occurrence and feeding habits of tonguefish (Cynoglossus lighti Norman, 1925) larvae were investigated in an offshore area (>5 m in depth) of the inner part of Ariake Bay, Japan. All specimens were symmetric, free‐swimming larvae. Although their seasonal abundance and distribution in the study site varied from year to year, spawning started in June and the larval abundance was high in August and September with a wide distribution in the inner part of the bay. Both present and previous study results strongly suggest that larvae may settle primarily in the estuary and near‐shore areas of Ariake Bay after their wide distribution in the offshore area of the bay during the free‐swimming stage. Larvae showed a clear feeding rhythm in which they fed on prey mainly during the daytime. Larvae fed exclusively on copepods, and identified prey were mostly Paracalanidae (mainly Parvocalanus crassirostris), Microsetella norvegica, and Oithona davisae. Pre‐metamorphosis larvae fed primarily on Paracalanidae and O. davisae, whereas O. davisae formed a smaller proportion of the early‐metamorphosis diet. In early metamorphoses, larvae fed preferentially on Paracalanidae and M. norvegica.     

Relationships between occurrences of toxic Dinophysis species (Dinophyceae) and small phytoplanktons in Japanese coastal waters

Fluctuations of the genus Dinophysis, which contained several toxic species of diarrhetic shellfish poisoning (DSP), were investigated during blooms in Hiroshima Bay, Mutsu Bay and Ise Bay, Japan. The co-occurrences of small phytoplanktons (cryptophytes, other nanophytoplanktons, cyanobacteria and eukaryotic picophytoplanktons) were investigated to search for relationships with mixotrophic Dinophysis. Cryptophytes were divided into three size-groups based on length of their chloroplasts (>10, 5–10 and <5 μm) during counting by epifluorescence microscopy. Clear relationships were not found between the occurrences of Dinophysis spp. and nanophytoplanktons, cyanobacteria and eukaryotic picophytoplanktons. However, the fluctuations of small-sized cryptophytes (<5 μm) showed a close relationship with that of D. acuminata in Hiroshima Bay. In Mutsu Bay, small-sized cryptophytes also accompanied the first occurrence peak of Dinophysis spp. In Ise Bay, peaks of the occurrences of middle- and small-sized cryptophytes were observed 2–3 weeks before the peak of D. acuminata. These cryptophytes decreased rapidly with increase in D. acuminata. These results suggest the possibility that small-sized cryptophytes may be food organisms for mixotrophic Dinophysis, with the abundance of Dinophysis dependent on these cryptophytes.     

Variation in the carbon content of two Asplanchna species

The rotifers of the genus Asplanchna were sampled four times during the summer from eight lakes of different types. The mean individual carbon content in the population varied between 0.15–0.66 µg C ind.^-1 (n = 21) for A. priodonta and 1.0–1.6 µg C ind.^-1 (n = 3) for A. herricki. The carbon content and the size of A. priodonta varied considerably between the populations of both different lakes and dates.The carbon level of both Asplanchna species (sample mean 0.2–1% of wet weight) was considerably lower than is generally found for rotifers. Much of the variation of carbon level could be explained by an inverse relationship with wet weight. The high variation in the carbon content of individuals suggests that Asplanch population may adapt their mean body size to fit prevailing environmental conditions.     

Ecological problems of Lake Ladoga: causes and solutions

We studied the outcome of competition between a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer species at five algal (Scenedesmus acutus) concentrations (0.5 × 10⁶ to 40.5 × 10⁶ cells ml^–1) and with varying initial densities in mixed populations (100 to 0% of B. calcyciflorus or A. fissa), the combined initial biomass being 0.2 µg ml^–1 in all test jars. Experiments were conducted at 28 ± 1 °C.Regardless of food concentration, B. calcyciflorus showed a greater increase in biomass than A. fissa, peak densities (mean ± standard error) at the lowest food concentration in the controls being 1.34 ± 0.31 µg dry weight ml^–1 and 0.82 ± 0.08 dry weight ml^–1, respectively. At the lower food concentrations, A. fissa displaced B. calyciflorus and vice versa at the higher food concentrations. At the intermediate food concentrations of 4.5 × 10⁶ cells ml^–1, B. calyciflorus outcompeted A. fissa only if its initial population density was three times higher. The rates of population growth in controls varied from 0.792 ± 0.06 d^–1 to 1.492 ± 0.13 d^–1 for B. calyciflorus and 0.445 ± 0.04 to 0.885 ± 0.01 for A. fissa depending on food level. When both species were introduced together, low food levels favoured higher abundance of A. fissa than B. calyciflorus, suggesting, in nature, it is likely that small Anuraeopsis colonize oligotrophic water bodies more successfully than larger Brachionus. The results also suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.     

Comparative carbon-specific ingestion rates of phytoplankton by Acartia tonsa,Centropages velificatus and Eucalanus pileatus grazing on natural phytoplankton assemblages in the plume of the Mississippi River (northern Gulf of Mexico continental shelf)

During four cruises in continental shelf waters of the northern Gulf of Mexico in the winters of 1981–83, we performed quantitative studies on the grazing of the copepods Acartia tonsa, Centropages velificatus, and Eucalanus pileatus, on phytoplankton using natural particulate assemblages as food. Stations were in, or adjacent to the plume of the Mississippi River, thereby providing wide spectra of phytoplankton and suspended riverine particulate concentrations. Phytoplankton cell volume was converted to carbon, and this, coupled with carbon content measurements of these three copepod species, allowed comparisons of daily ingestion effort even though the copepods were of different sizes. Data were expressed in the same units (% of copepod body carbon ingested copepod ^–1 d^–1) for each species. Over similar ranges of phytoplankton carbon concentrations (0.21–92.06 gCl^–1), Acartia tonsa had higher carbon-specific ingestion rates (x = 22.31%, range = 0.08–152.37%) than C. velificatus (x = 2.8%, range = 0.00–31.09 %) or E. pileatus (x = 1.27%, range = 0.10–2.80%). Carbon-specific ingestion rates increased with increasing phytoplankton carbon concentration for A. tonsa (R² = 0.81) and there was no evidence of saturated feeding on the carbon concentrations offered. A similar, but weaker trend was evident for E. pileatus (R² = 0.71), but not C. velificatus (R² = 0.49). Over a wide range of suspended particulate concentrations (10.6–95.2 mg l^–1), there was no systematic effect of particulates on carbon-specific ingestion rate for any of the three copepod species. However, A. tonsa appeared more adept at grazing in highly turbid water than C. velificatus or E. pileatus.     

Note on the occurrence of Pseudo-nitzschia australis and domoic acid in squid from Monterey Bay, CA (USA)

Over the past decade diatom blooms of domoic acid (DA)-producing Pseudo-nitzschia spp. have been responsible for numerous marine mammal and bird mortalities in Monterey Bay, CA. One possible toxin vector is the market squid, Loligo opalescens, a small pelagic mollusk that plays an important role in the near-shore food web of the California Current ecosystem as a favored vertebrate prey species. This study examined the trophic link between toxic Pseudo-nitzschia and L. opalescens using toxin and stomach content analyses of animals collected from Monterey Bay, CA in 2000. Receptor binding assay data (confirmed by tandem mass spectrometry), demonstrated the presence of DA in squid during a toxic Pseudo-nitzschia event, with P. australis frustules observed in stomach samples. Though DA levels were low (<0.5 μg DA g⁻¹ tissue) in L. opalescens during the study period, it is now clear that this potent neurotoxin can occur in squid and is likely delivered through its krill prey species, which are known to retain DA after feeding on toxic Pseudo-nitzschia. Our findings suggest that further study of the relationship between Pseudo-nitzschia blooms and DA contamination of squid is warranted to better evaluate the potential health risk to humans and wildlife associated with this major commercial seafood species and important prey item.     

Distribution, abundance, behavior and metabolism of Periphylla periphylla, a mesopelagic coronate medusa in a Norwegian fjord

The distribution, behavior and metabolism of the mesopelagic jellyfish, Periphylla periphylla (Péron & Lesueur), were investigated in Lurefjorden, Norway. Field studies, conducted in 1998–1999 with plankton nets and a remotely operated vehicle, indicated that 80-90% of the dense (up to 2.5 m^–3) population migrated 200–400 m vertically each day throughout the year. In situ observations with red light revealed that swimming rates and feeding activity varied with age and time of day. Detection of turbulence and contact with surfaces caused this medusa to conceal one or all of its tentacles in the stomach or to shed nematocyst-laden tissue from the tentacles. Stomachs of medusae collected with nets were often full of prey entangled with the sloughed tissue. Stomachs of medusae captured individually with ROV samplers were empty or contained only a few prey in their stomachs (typically, 1–4 copepods Calanus spp. or chaetognaths Eukrohnia hamata Möbius per medusa). Low rates (0.4–5.6 l O₂ mg C^–1 h^–1) of oxygen consumption of P. periphylla suggested that this species was sustained by relatively few (1–34) prey d^–1.     

Feeding behaviour and morphology of filtering combs of Daphnia galeata

The population of Daphnia galeata Sars from the fish pond Velký Pálenec (Blatná, Czechoslovakia) living in high food conditions (7 mgC l^–1) was characterized by a small size of the filtering comb on the thoracic limb 3, measured as seta length, length of the base of the comb and number of setae (population 1). One month cultivation of this population in low food conditions (1.5 mgC l^–1) in the laboratory resulted in twofold increase in size of the filtering comb (population 2). Filtering and ingestion rates of both populations were measured at eight concentrations of food (approximately 0.025–3.2 mgC l^–1) using ¹⁴C labeled Scenedesmus acutus. The results show that size of the filtering combs influences considerably feeding behavior of Daphnia. The comparison of animals with the same body length suggests that the population with a large comb feeds at concentration of food below 0.4 mgC l^–1 more intensively and reaches the maximum of the filtering rate at a lower concentration than the population with a small comb. The situation is opposite at concentration above 0.4 mgC l^–1. The higher values of theoretical flow in population with a small projection of filtering area suggest that this population has to compensate disadvantage of a small comb with the higher appendages beat frequency.     

Diet and prey selection in larval and juvenile Japanese anchovy Engraulisjaponicus in Ariake Bay, Japan

We studied the diet of larval and juvenile Japanese anchovy Engraulis japonicus in the upper Ariake Bay, Japan. Diet was analyzed by examining the digestive tracts; feeding intensity, proportion of empty guts, and prey selectivity were calculated. Anchovy density was negatively influenced by temperature and positively by salinity and prey density. Diet was dominated by Acartia omorii, which was positively selected with two other copepods, Calanus sinicus and Pseudodiaptomus marinus. In contrast, Oithona davisae was highly dominant in the environment but was absent in anchovy guts; thus, this copepod was negatively selected, with two others, Tortanus derjugini and Sinocalanus sinensis. Overall, larger prey were positively selected and smaller ones were negatively selected; value of electivity index correlated negatively with prey size. Larvae [<18 mm of standard length (SL)] showed significantly lower feeding intensities and higher rates of empty guts than juveniles (≥18 mm SL). In juveniles, feeding intensity increased steadily as the fish grew in size, with a corresponding reduction in empty guts. Feeding intensity correlated positively and empty gut correlated negatively with fish size. We suggest that larger prey are important diets for postlarval Japanese anchovy in Ariake Bay.     

Competitive advantage of the invasive copepod Oithona davisae over the indigenous copepod Oithona nana in the Marmara Sea and Golden Horn Estuary

Around 2010, the alien copepod Oithona davisae was introduced to the Marmara Sea and Golden Horn Estuary to live alongside the native species, Oithona nana. By October 2015, the population number, including nauplii, of the introduced species (213,600 ind m^?3) was higher than that of the native species (54,900 ind m^?3) and close to that of O. davisae in Sevastopol Bay of the Black Sea (342,000 ind m^?3) where it appeared 14 years ago. A high percentage of O. davisae females with ovisacs (up to 30% of total female number) carrying up to 15.1 ± 2.1 eggs in the sac and the predominance of nauplii in the population (up to 95% of the total population number) indicate that this alien copepod has become a permanent component of the Marmara Sea and Golden Horn Estuary ecosystem. Our experiments showed that in comparison with widely euryhaline O. davisae, O. nana is a stenohaline species. It has a salinity tolerance in the range of 13–27 psu and, in spite of having a larger body, a lower body density (1.037 ± 0.007 g cm^?3) and sinking speed. Carbon-specific respiration rates of both species were close (0.32–0.38 μL O₂ μgC^?1 day^?1) and similar to that of O. davisae from the Black Sea during this period but the frequency and speed of routine jumps, and the maximum escape reaction speed (up to 23.3 ± 1.7 cm s^?1) in O. davisae from the Marmara Sea were significantly higher. This indicates the higher adaptive potential of O. davisae in this situation compared to that of O. nana.     

A new species of Hemicyclops (Copepoda, Poecilostomatoida) from burrows of the mud shrimp Upogebia major in an estuarine mud-flat in Tokyo Bay, Japan

A new species of poecilostomatoid copepod, Hemicyclops tanakai n. sp. was collected from burrows of the mud shrimp Upogebia major in an estuarine mud-flat in Tokyo Bay. The new species can be distinguished from its congeners by combination of the following characteristics: setation of the antennule, the segmentation of the antennule, the length–width ratio of the caudal ramus, the ornamentation on the third segment of antenna and the shape of the genital double somite.     

Field energetics of a large carnivorous lizard,Varanus rosenbergi

Summary Field metabolic rates (FMRs; CO₂ production) and water influx rates of Varanus rosenbergi were measured seasonally by means of doubly-labelled water (³H₂ ¹⁸O). Metabolic rates and water influexes were highest in summer (0.18 ml CO₂ g^–1 h^–1 and 17.9 ml H₂O kg^–1 day^–1) and lowest in winter (0.04 ml CO₂ g^–1 h^–1 and 6.5 ml H₂O kg^–1 day^–1). FMRs and water fluxes were intermediate in spring. It is estimated that a 1-kg V. rosenbergi would need to consume 4.7 kg of prey each year to remain in energy balance and that the energy commitment to egg production requires a 40% increase in the food acquisition of a gravid female in late spring and early summer. It is calculated that water influxes in summer are totally provided from food but in other seasons, particularly winter, pulmocutaneous water exchange and/or drinking are significant avenues of water intake.     

Food and daily food consumption of southern minke whales in the Antarctic

Summary The stomach contents of 273 southern minke whales (Balaenoptera acutorostrata) taken in the region 55°S to the ice-edge and between 105°E and 115°E by a Japanese survey during 1987/88 were examined. The minke whales' dominant feeding ground coincided with a distinctive hydrographic front in the vicinity of the ice-edge at the mouth of Vincennes Bay. Krill (Euphausia superba) were the dominant food species comprising 100% and 94% by weight of stomach contents in the ice-edge and offshore zones, respectively. In the offshore zone, minke whales tend to feed on E. superba rather than Thysanoessa macrura, which is found more frequently than the former in net samples. Total food consumption by minke whales per day was estimated to be 1170 t (22.1 kg/km²) and 596 t (2.0 kg/km²) in the ice-edge and offshore zones, respectively. Feeding activity peaked in the early morning in the ice-edge zone, whereas it occurred irregularly throughout the day in the offshore zone. Two size modes of krill (25–28 mm in body length (1 year-old) and 41–48 mm (3–4 year-old)) dominated the diet. Although the former was numerically more abundant, the latter dominated the diet by weight, suggesting that larger krill are more important food for minke whales. Observed spatial pattern of krill populations in the study region suggests that movement of the subsurface cold water mass tended to carry krill offshore from the ice-edge.The earlier version of this paper was submitted to the 42nd Meeting of the Scientific Committee of International Whaling Commission as SC/42/SHM; 34.     

Biosynthesis of protease by Pseudomonas aeruginosa MN7 grown on fish substrate

Fish powders and fish protein hydrolysates (FPH) from sardinella (Sardinella aurita) were prepared and tested as growth media for alkaline protease production by Pseudomonas aeruginosa MN7. Cultivated in fish substrate as carbon source, the strain exhibited a slightly greater protease production (about 7800 U ml^–1) than that obtained with commercial peptones (about 7222 U ml^–1). Furthermore, P. aeruginosa MN7 produced the same amount of protease when cultivated in medium containing only fish substrate or that containing all ingredients, indicating that the strain can obtain its carbon and nitrogen requirements directly from whole fish proteins. Moreover, it was found that extensive hydrolysis of fish proteins did not increase protease formation. Protease production in media containing only FPH prepared by Alcalase was about 70% of those obtained with MN7 protease digest of fish protein or with meat-fish powder. These results indicate that sardinella substrates are an excellent carbon and nitrogen source for the growth of P. aeruginosa MN7 and the production of protease.     

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.