Cadmium in Halobates micans from the central and south Atlantic Ocean

The concentrations of cadmium in 428 samples of the sea-skater Halobates micans from 57 locations in the central and south Atlantic Ocean ranged from 1.7 to 122 g Cd g^-1 dry wt. The concentrations in females, males and nymphs were not significantly different. The data suggest that the cadmium concentrations in these insects are somehow related to surface currents. The highest values (50 to 70 g g^-1, maximum value 122 g g^-1) were recorded from the South Equatorial Current region; intermediate values (10 to 20 g g^-1) from the Equatorial Counter Current and the North Equatorial Current; and relatively low concentrations (>10 g g^-1) from the Canary Current in the north and the Benguela Current in the south. A remarkable feature of these data lies in the fact that they reveal areas of high Cd content which correspond closely with those indicated by data obtained by Bull et al. (1977) from a different set of samples, analyzed by different methods and obtained in different years.     

Anatomy of structures associated with air-breathing in Orchestia gammarellus (Crustacea: Amphipoda: Talitridae): coxal plates and gills

The structure of the coxal gills and coxal plates of the semi-terrestrial beachflea Orchestia gammarellus (Pallas) (Crustacea: Amphipoda: Talitridae) is described in relation to their possible use for aerial gas exchange and ion exchange. Anatomical evidence is presented to support the hypothesis that the medial surface of the coxal plates functions as an extrabranchial aerial gas-exchange site in O. gammarellus. Thus, the effective diffusion distance across the medial (or inside-facing) surface of O. gammarellus coxal plates (mean±SD=5.4±0.3 m; n=9, cuticle thickness 4.4±0.5 m, n=21) is only a third of the equivalent distance across both the coxal gills (18.4±6.0 m, n=10; cuticle thickness 1.7±0.6 m, n=7) and the lateral (or external) surface of the coxal plates (19.4±0.7 m, n=5; cuticle thickness 8.7±0.8 m, n=7). Chloride-ion-permeable areas were located using a silver-staining technique. All ten coxal gills appeared to be equally permeable to chloride ions after examination with a light microscope. However, the coxal plates and the rest of the integument do not appear to be chloride-permeable.     

The functions of octopine dehydrogenase and D-lactate dehydrogenase in the pedal retractor muscle of thedog whelk Nassarius coronatus (Gastropoda: Nassariidae)

The pedal retractor muscle of Nassarius coronatus, unlike most mollusc muscles, contains high activities of both octopine dehydrogenase (159 IU g^-1 wet wt muscle) and D-lactate dehydrogenase (40 IU g^-1 wet wt muscle). Pedal retractor muscles show accumulation of octopine (increase of 4.7 mol g^-1 wet wt muscle), and a fall in arginine phosphate concentration (decrease of 4.1 mol g^-1 wet wt musscle) after exercise. During recovery from exercise, octopine returns to resting levels after about 120 min, D-lactate accumulates (5.1 mol g^-1 wet wt muscle), and arginine phosphate returns to resting levels within 30 min. D-lactate (2.0 mol g^-1 wet wt muscle) accumulates during long-term exposure to air. The results show that octopine dehydrogenase and D-lactate dehydrogenase can function to catalyze the terminal step of glycolysis during muscle anoxia associated with these different physiological states.     

Metabolic characteristics of midwater zooplankton: Ammonia excretion,O:N rations,and the effect of starvation

The nature of protein catabolism in a wide range of species of midwater zooplankton was investigated. The weight-specific ammonia excretion rates (g NH₃–N g^–1 dry wt h^–1, y) decline exponentially with minimum depth of occurreece (MDO, x), y=163.4 x^{–0.479±0.212} (95%ci) (CI=confidence interval), when temperature is held constant. The change in ammonia excretion can be partially explained by the decrease in percent protein (%P) with MDO, %P=80.17 MDO^{–0.148±0.122 (95%ci)} The atomic O:N ratio of freshly caught zooplankters ranged from 9.1 to 91, with most measurements between 9 and 25. Detailed studies were carried out on the response of one of the species studied (Gnathophausia ingens) to starvation (28 d). After 14 d of starvation the average ammonia excretion rate declined by more than 75% to less than 1 g NH₃–N g^–1 wet wt h^–1, although the average oxygen consumption declined by only 13% within the first 7 d of starvation and then remained stable. This differential response of oxygen consumption and ammonia excretion to starvation resulted in an increase in the average O:N ratio of starved animals from an initial 33 to 165 after 21 d. The average O:N ratios of fed mysids remained below 38 during the experiment. G. ingens maintains a relatively uniform metabolic rate during starvation by relying more heavily on its large lipid stores than when being fed.     

Occurrence of tri-n-butyltin-caused imposex in the North Pacific marine snail Nucella lima in Auke Bay,Alaska

Like female dog whelks (Nucella lapillus) in the Atlantic Ocean, females of the Pacific gastropod N. lima respond to low concentrations of tri-n-butyltin (TBT) by growing a penis and vas deferens. This condition, termed imposex, was found to occur in N. lima collected from August 1987 to May 1988 along a TBT pollution gradient associated with a marina in Auke Bay, Alaska. The suite of symptoms characteristic of imposex in N. lima was slightly different than in N. lapillus. Imposex, as measured by relative penis size (RPS) of females to males, increased from 0.0 to 34.27 along this gradient and as measured by vas deferens sequenceindex (VDS) increased similarly from 0.0 to 4.29. Concentrations of TBT in N. lima increased from below detection limits (about 0.010 g Sn g^-1 dry tissue wt) to 0.065 g Sn g^-1 dry tissue wt along the gradient. The gradient was determined by measuring TBT in whole-body tissues of bay mussels (Mytilus edulis). Concentrations of TBT in mussels increased from below detection limits to 0.833 g Sn g^-1 dry tissue wt in mussels from within the marina that was the major local source of TBT. Imposex, tissue TBT burden, and position along a TBT pollution gradient are significantly correlated (P<0.01) in N. lima. TBT was tested as a causative agent for imposex by exposing snails at a distant control site to TBT paint. After 1 mo exposure, 33% of the females grew a penis ranging in size from 0.2 to 0.8 mm. Our results generally corroborate those found for N. lapillus and indicate that imposex in N. lima is caused by environmental TBT exposure. We suggest that the RPS in the genus Nucella may be useful in monitoring TBT in coastal waters worldwide.     

A role forAmanita muscaria L. in the circulation of cadmium and vanadium in a non-polluted woodland

The sporophore of the fungusAmanita muscaria L. contains greatiy elevated levels of cadmium (29.9 g g^–1 dwt) and vanadium (344.9 g g^–1 dwt) in comparison with the soil in a birch woodland (total (HNO₃-extractabie Cd 0.4 g g^–1 dwt, V 11.7 g g^–1 dwt). The significance of this remarkable concentration of normally rare and dispersed elements in terms of their circulation in the woodland has been investigated. Both elements are released from sporophore tissue in a form which can be taken up by a test plant (lettuce), cultivated in the woodland soil amended with different quantities of sporophore tissue, Cadmium levels in all plant tissues were elevated in comparison to the non-amended controls; only root vanadium levels responded to the amendment of the soil. The results are discussed in terms of their significance for the natural cycling of both elements. It is calculated that an abundant population of sporophores could circulate 1.4% of the total cadmium and 0.65% of the total vanadium pool found in the litter layer and 0–5 cm soil horizon in the sampled woodland over a period of 14 days (mean life span of a sporophore).     

Carotenoids in the tiger prawn Penaeus esculentus during ovarian maturation

Female Penaeus esculentus Haswell were collected by 15 to 20 min duration trawls during 1990. Carotenoids were analysed in the digestive gland, abdominal muscle, the remainder of the body (hereafter called integument) and ovary of prawns in Stage 2 through Stage 4 (fully mature) of maturation. The only oxycarotenoids (xanthophylls) identified were astaxanthins or astaxanthin esters; occasionally low levels of -carotene were detected in the digestive gland. The concentrations of astaxanthin monoesters (AM) and diesters (AD) were highest, with only minor amounts of free astaxanthins (Ast), except in the maturing ovaries, where free astaxanthins predominated (up to 80% of the total carotenoid). Of the total carotenoid, 82 to 94% was in the integument, but at maturity the digestive gland contained 10.7±3.4% and the ovary 5.6±0.9% of the total carotenoid. Only the ovary increased in mass during maturation, reaching up to 5.2% of total prawn mass. During this period, digestive gland concentrations of AM, AD and Ast all increased (tota 20 to 120 g g^-1); levels in the muscle and integument varied little throughout maturation (total 0.4 and 100 g g^-1, respectively); ovary AM levels remained low throughout (1.5 to 1.2 g g^-1), AD increased from only 2 to 5 g g^-1, but Ast increased from 2 to 34 g g^-1. Apart from the ovary, AM concentrations were the most variable. In common with other decapod Crustacea, the maturing ovary of P. esculentus contained high levels of carotenoids, indicating that these may have an important role in early development. The natural diet of P. esculentus includes a variety of carotenoids, but except for a little -carotene, the digestive gland, where absorption occurs, contained astaxanthins, with only an occasional trace of -carotene. This suggests that the conversion of dietary carotenoids to astaxanthin occurs soon after ingestion.     

Seasonal photosynthetic characteristics of Ascoseira mirabilis (Ascoseirales,Phaeophyceae) from King George Island,Antarctica

Monthly variation in photosynthesis, dark respiration, chlorophyll a content and carbon: nitrogen (C:N) ratios in different lamina sections of adult plants of Ascoseira mirabilis Skottsberg from King George Island, Antarctica, was investigated between September 1993 and February 1994. Light saturated net photosynthesis (P _max) showed maximum values in September (12 to 25 mol O₂ g^-1 fr wt h^-1), and decreased towards the summer to values ranging between 2.0 and 5.0 mol O₂ g^-1. In the distal section, however, a second optimum occurred in December (25 mol O₂ g^-1 fr wt h^-1). Dark respiration rates were also highest in October and November and decreased strongly in December to February (6.0 and 1.0 mol O₂ g^-1 fr wt h^-1, respectively). Gross photosynthesis exhibited high values between September and December. Concomitant with the seasonal decrease of photosynthetic efficiency () from mean values of 1.2 mol O₂ g^-1 fr wt h^-1 (mol photons cm^-2 s^-1)^-1 in September to 0.3 mol O₂ g^-1 fr wt h^-1 (mol photons cm^-2 s^-1)^-1 in January, the initial light saturating point (I _k) gradually increased from 19 to 60 mol photons m^-2 s^-1. Likewise C:N ratios were low in spring (12 to 13) and increased in summer (20). In general, the photosynthetic parameters P _max, gross photosynthesis, and Chl a concentrations were significantly higher in the distal section of the thallus. In contrast, C:N ratios were lower in the distal section of the lamina. The results show that photosynthesis obviously strongly supports growth of the alga in late winter to spring, as it does in some morphologically related brown algae from temperate and polar regions. The question whether growth is additionally powered     

Uptake of cadmium by the eelgrassZostera marina

The uptake of cadmium by the shoots ofZostera marina L. (eelgrass) was examined in the laboratory. Experiments were carried out in experimental chambers which allowed the separation of the leaves from the root-rhizome portions of intact shoots. Cadmium uptake by the root-rhizome portions over 24 h was directly related to substrate cadmium concentration (1.0, 5.0 and 10.0 g Cd ml^-1) and varied from 6.5 to 30.0 g Cd g^-1. Cadmium uptake by the root-rhizomes and the leaves in a substrate concentration of 1 g Cd ml^-1 was also related to exposure times (24, 48 and 72 h). Maximum uptake by the root-rhizomes and the leaves was observed after 72 h at a substrate concentration of 1 g Cd ml^-1 and was equivalent to 48 and 94 g Cd g^-1, respectively. Translocation of cadmium from the leaves to the root-rhizomes was observed after 24 h, and at the end of 72 h was equivalent to 27% of the total leaf uptake. No cadmium movement from the root-rhizomes to the leaves was detected.     

Cadmium in ocean-skaters,Halobates sericeus (Insecta), and in their seabird predators

Between February 1978 and February 1981, the content of cadmium was measured (1) in specimens of Halobates sericeus picked out from regurgitates of 9 species of seabirds, and (2) in the feathers of some seabirds which nested on 8 islands or atolls in the Northwestern Hawaiian Islands. In the 4 major avian predators of this oceanskater, the Cd concentrations increased from roughly 2 to 8 g g^-1 dry wt towards the northwest, over a linear distance of about 1500 km. The correlation between Cd levels in the insects and in the feathers of their predators suggests that this heavy metal is transmitted along the food chain.     

Carotenoids versus retinoids (Vitamin A) as essential growth factors in penaeid prawns (Penaeus semisulcatus)

Penaeus semisulcatus de Haan adult gravid females, eggs and non-feeding early larvae, including Protozoea I, were used to test the hypothesis that retinoid (Vitamin A) is not required in early decapod crustacean development. In the adult gravid females, retinoids were detected only in the eyes (1.56±0.23 g g^-1 wet mass), whereas there were up to 97 g g^-1 wet mass total carotenoid in digestive gland and epidermis. This was mostly esterified, except in the ovaries, where free astaxanthin predominated (30 g g^-1). No retinoids could be detected in the eggs, the naupliar stages or Protozoea I, but free astaxanthin was metabolised exponentially, falling from 19 g g^-1 in the eggs to 4 g g^-1 in Protozoea I. This suggests that retinoid is not essential in early development and that carotenoid could be taking its place. Also, including retinoids in artificial diets appears to be unnecessary, provided adequate carotenoid is supplied.     

Light requirements for growth of six shade-acclimated Mediterranean macroalgae

Six mediterranean macroalgae were cultivated for more than 2 yr under shade culture conditions, after which light requirements for growth were investigated at 16±2°C. The saturation light levels for growth in the logarithmic phase were related to the bathymetric distribution of the algae on the shore. The eulittoral to supralittoral red alga Bangia atropurpurea was saturated at a photon fluence rate of 71 mol photons m^-2 s^-1, the upper sublittoral to eulittoral brown algae Scytosiphon lomentaria, Colpomenia peregrina and Kuckuckia spinosa and the sublittoral brown alga Stictyosiphon soriferus at 39 to 71 mol photons m^-2 s^-1, and the deep-water alga Choristocarpus tenellus at 19 mol photons m^-2 s^-1. The minimum light requirements for growth of B. atropurpurea and C. tenellus were determined by observing length increase for 56 d under limiting light conditions. The compensation and minimum irradiances required for growth of B. atropurpurea were 0.5 and 1 mol photons m^-2 s^-1 respectively. The corresponding values for C. tenellus were 0.15 to 0.28 and 0.5 mol photons m^-2 s^-1 respectively. C. tenellus was the siowest-growing species tested at saturating light conditions, but it grew faster than B. atropurpurea at 1 mol photons m^-2 s^-1. Both B. atropurpurea and C. tenellus were able to survive 56 d in darkness, but only the latter grew under darkness in the first 14 d.     

Comparison of the scope for growth with the growth performance of Ostrea edulis seed reared at different food concentrations in an open-flow system

Oyster (Ostrea edulis L.) seed was reared on five rations of Isochrysis galbana Parke: 10, 30, 100, 200 and 300 cells l^-1, in an open-flow system. Physiological parameters such as clearance, ingestion, absorption and respiration rates were measured, and the scope for growth (SFG) calculated from these parameters was compared with actual growth over 20 d. Actual growth was negligible at 10 cells l^-1 (daily maintenance ration=2.5% organic wt) and maximum (growth rate=13.64 d^-1) at 200 cells l^-1 (daily ration=43% organic wt), which also supported the maximum gross and net growth efficiencies (K ₁=50%, K ₂=85%). Ingestion rate was directly correlated to cell concentration up to maximum of 100 cells l^-1, but further increases failed to support higher ingestions. Absorption efficiency decreased with increasing algal concentration from 95% down to 60%. Due to the costs of growth, respiration rate significantly increased when food was added above maintenance levels; however, metabolic costs associated with feeding activity were undetectable. The scope for growth calculated from these physiological parameters agreed with the long-term growth performance, validating the physiological energetics method as a valuable tool for predicting long-term growth performance under constant environmental conditions.     

Biochemical composition of Nyctiphanes australis and its possible use as an aquaculture feed source: Lipids,pigments and fluoride content

Nyctiphanes australis contained, on a dry weight basis, an average of 52% crude protein and 5.0 to 9.5% lipid. The fatty acid profile of N. australis was markedly unsaturated, with a mean total 3 fatty acid content of 48.6±2.4% of total fatty acids. N. australis contained high levels of the essential long-chain polyunsaturated fatty acids eicosapentaenoic (EPA, 20:53) and docosahexaencic (DHA, 22:63), ranging from 16.6 to 36.5% and 11.1 to 24.8%, respectively. The concentration of total carotenoids ranged from 137 to 302 g g^–1 dry wt, with no significant differences in concentrations found with season or life stage. The carotenoids were comprised of 79.5% astaxanthin and 20.5% canthaxanthin. The lipid and pigment compositions of N. australis suggest that the species could serve as a suitable feed source for cultured salmonids. Like other euphausiids, N. australis contained high levels of fluoride, with a seasonal range between 277 and 3507 g g^–1 dry wt. The high fluoride levels found in N. australis would not detract from its potential as a feed source for salmonids because ingested fluoride is largely absorbed by the skeleton.     

Seasonal and depth related variation in the photosynthesis–irradiance response of<Emphasis Type="Italic"> Ecklonia radiata</Emphasis> (Phaeophyta,Laminariales) at West Island,South Australia

The photosynthesis–irradiance response of Ecklonia radiata (C. Agardh) J. Agardh, a common kelp in the temperate southern hemisphere, was investigated in situ throughout the year and across a depth profile at West Island, South Australia. Temperature and irradiance environment altered throughout the year, varying at 3 m between 14–20°C and 279–705 mol photons m^–2 s^–1. Photosynthetic capacity (P_m) varied throughout the year between 177–278 mol O₂ g^–1 dry wt h^–1 at 3 m and 133–348 mol O₂ g^–1 dry wt h^–1 at 10 m. The irradiance required for sub-saturation of photosynthesis (E_k) varied between 97–152 and 81–142 mol photons m^–2 s^–1 for 3 m and 10 m respectively, and the respiration rate varied between 15–36 and 13–20 mol O₂ g^–1 dry wt h^–1 for 3 m and 10 m. A clear seasonal change in photokinetic parameters was detected and provided strong evidence for a seasonal acclimation response. During winter an increase in the efficiency of light utilisation at low irradiance () was accompanied by a decrease in both E_k and that required for photosynthetic compensation. P_m also increased during the winter and autumn months and respiratory requirements decreased. These changes enable E. radiata to display an optimal photosynthetic performance throughout the year despite significant changes in the surrounding environment.Communicated by P.W. Sammarco, Chauvin     

Concentrations of Mn,Fe, Cu,Zn and Cd in the mesopelagic decapod Systellaspis debilis from the East Atlantic Ocean

Specimens of the oceanic decapod Systellaspis debilis were collected from six sites in the East Atlantic Ocean between 1970 and 1984, and were analysed for Mn, Fe, Cu, Zn and Cd. The data confirm that there are small but significant differences in mean metal concentrations from some sites which showed no obvious pattern in relation to geographic location of the samples. As a result, ranges of site means are quoted as baseline levels for each metal (g g^-1 dry wt): 2.3 to 2.9 g Mn g^-1, 31.2 to 77.8 g Fe g^-1, 25.9 to 83.4 g Cu g^-1, 41.9 to 92.9 gZn g^-1, 11.1 to 31.8 g Cd g^-1. The concentration of cadmium in S. debilis from all sites was raised relative to cadmium concentrations reported for coastal decapods, perhaps as a result of dietary enrichment. Metal accumulation may provide useful information for understanding the complex feeding behaviour of many oceanic animals.     

Changes in the digestive gland of Euphausia superba during short-term starvation: lipid class,fatty acid and sterol content and composition

During a period of short-term (19 d) starvation, total lipid in the digestive gland of Euphausia superba Dana decreased from 21 to 9% dry weight. Total lipid per digestive gland decreased significantly during starvation compared to Day 0 individuals, falling from 1960 (±172) to 385 (±81) g. Polar lipid was the major lipid class utilised during starvation, falling from 1510 (±225) to 177 (±46) g per digestive gland (76 to 45%). Absolute levels of triacylglycerol fell from 300 (±41) to 76 (±5) g; however, relative levels remained unchanged. The relative level of free fatty acid increased significantly with starvation (4 to 39%) with absolute levels ranging from 79 (±1) to 156 (±20) g per digestive gland. Absolute levels of all fatty acids per digestive gland declined continually until the end of the starvation period. The long-chain polyunsaturated acids eicosapentaenoic (20:53) and docosahexaenoic (22:63), decreased with starvation from 37 to 26% and 15 to 10%, respectively whereas the saturated fatty acid, palmitic acid (16:0), increased from 15 to 20%. Cholesterol, the major sterol in this organ, increased from 17 (±20) to 44 (±13) g per digestive gland by Day 3, and by Day 19 had returned to levels found in the digestive gland of Day 0 individuals. Desmosterol followed a similar pattern to cholesterol, increasing from 3 (±1) g per digestive gland on Day 0 to 11 (±4) g on Day 3, and falling to 2 (±1) g on Day 19. Other sterols in the digestive gland, predominantly of algal origin, fell from the levels found in Day 0 individuals to near zero amounts by Day 6. The digestive gland of E. superba plays a dynamic role during shortterm starvation in terms of lipid content and composition. The relative levels of polar lipids, free fatty acids and cholesterol in the digestive gland may provide reliable indices of the nutritional condition of E. superba in the field. Sterols in the digestive gland are indicative of recent dietary composition of krill, and may also be used to quantify dietary input from individual phytoplanktonic species.     

Regulation of growth in Laminaria digitata: use of in-vivo nitrate reductase activities as an indicator of nitrogen limitation in field populations of Laminaria spp.

The seasonal variation in growth rate of a population of Laminaria digitata (Huds.) Lamour growing at Arbroath, Scotland was studied between August 1981 and September 1982, and was found to follow the biphasic annual cycle typical of this genus. Growth rates were maximum (0.3 cm cm^-1 mo^-1) in early June and minimum (0.05 cm cm^-1 mo^-1) between September and January. An analysis of the relationship between the seasonal changes in environmental factors (inorganic nitrogen concentrations, irradiance and temperature) with those of growth rate and the accumulation or mobilisation of cellular reserves of carbohydrates and nitrate, indicated that growth was nitrogen-limited between June and October and light-limited (with a possible co-involvement of temperature) for the remainder of the year. These conclusions were supported by the seasonal changes in the ratio of actual: potential in-vivo nitrate reductase activities in L. digitata, thus confirming the suitability of this technique for monitoring the occurrence of nitrogen limitation in Laminaria spp. The seasonal changes in blade nitrate reductase activities closely followed those of growth rate, with maximum activities [0.3 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] being present in late May and minimum levels [0.01 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] occurring between November and March. The correlation observed between nitrate reductase activities and growth rate is consistent with the ability of Laminaria spp. to store excess inorganic nitrogen, available during winter and early spring, as NO ₃ ^- , and with the requirement to conserve enzyme protein during the summer period of nitrogen limitation.     

The ammonium/methylammonium uptake system of Symbiodinium microadriaticum

The substrate analogue [¹⁴C]-methylammonium was used to study ammonium/methylammonium uptake by Symbiodinium microadriaticum (zooxanthellae). The value of the Michaelis constant (K _m) for the uptake system was approximately 35 M with methylammonium as substrate; ammonium was a competitive inhibitor of methylammonium uptake, and the K _m for ammonium uptake (determined as the inhibition constant, K _i, for methylammonium) was 6.6 M. Methylammonium uptake by zooxanthellae was light-dependent. Methylammonium uptake rates of zooxanthellae which had been freshly isolated from the hermatypic coral Acropora formosa (0.85±0.05x10^-10 mol min^-1 cell^-1) were lower than those of axenic cultures of the zooxanthellae from Montipora verrucosa (Acroporidae) grown under various nitrogen regimes (1.6 to 12x10^-10 mol min^-1 cell^-1). Maximum uptake rates were found for ammonium-starved cultured M. verrucosa zooxanthellae (10.2 to 12x10^-10 mol min^-1 cell^-1); M. verrucosa zooxanthellae growing with ammonium as nitrogen source and zooxanthellae which had been freshly isolated from A. formosa gave similar and considerably lower uptake rates (0.85 to 1.6x10^-1 mol min^-1 cell^-1). These results suggest that either coral tissue contains sufficient ammonium to repress synthesis of the uptake system of the algal symbionts or, alternatively, there are additional barriers to ammonium transport for zooxanthellae in vivo.     

Ammonium excretion by gelationous zooplankton and their contribution to the ammonium requirements of microplankton in Chesapeake Bay

Ammonium excretion rates of recently collected specimens of gelatinous zooplankton, the scyphomedusan Chrysaora quinquecirrha DeSor and the etenophore Mnemiopsis leidyi A. Agassiz, were correlated with body mass and water temperature in measurements made from April to October 1989 and 1990. Rates ranged between 3.5 and 5.0 g atoms NH ₄ ⁺ -N (g dry wt)^-1h^-1 for C. quinquecirrha and 3.0 to 4.9 g atoms NH ₄ ⁺ -N (g dry wt)^-1h^-1 for M. leidyi. Excretion rate equations and in situ data on the size distributions and biomasses of gelatinous zooplankters and water temperature were used to estimate the contribution of ammonium by medusae and ctenophores to mesohaline Chesapeake Bay waters on several dates during April to October 1989 and 1990. We then compared the estimated contributions to direct measurements of ¹⁵NH ₄ ⁺ uptake by microplankton. The maximum estimated regeneration by gelatinous zooplankton was 5.8 g atoms NH ₄ ⁺ -N m^-3h^-1 at night in August 1990, when medusae biomass was greatest. This represents about 4% of the ammonium required by the microplankton. During the daytime on all dates, less than 1% of the ammonium required by microplanktion was supplied by gelatinous zooplankton. Therefore, gelatinous zooplankton appear to play a minor role in the ammonium cycle of Chesapeake Bay.