Microbial activity and carbon, nitrogen, and phosphorus content in a subtropical seagrass estuary (Florida Bay): evidence for limited bacterial use of seagrass production

Bacterial abundance, production, and extracellular enzyme activity were determined in the shallow water column, in the epiphytic community of Thalassia testudinum, and at the sediment surface along with total carbon, nitrogen, and phosphorus in Florida Bay, a subtropical seagrass estuary. Data were statistically reduced by principle components analysis (PCA) and multidimensional scaling and related to T. testudinum leaf total phosphorus content and phytoplankton biomass. Each zone (i.e., pelagic, epiphytic, and surface sediment community) was significantly dissimilar to each other (Global R = 0.65). Pelagic aminopeptidase and sum of carbon hydrolytic enzyme (esterase, peptidase, and α- and β-glucosidase) activities ranged from 8 to 284 mg N m⁻² day⁻¹ and 113–1,671 mg C m⁻² day⁻¹, respectively, and were 1–3 orders of magnitude higher than epiphytic and sediment surface activities. Due to the phosphorus-limited nature of Florida Bay, alkaline phosphatase activity was similar between pelagic (51–710 mg P m⁻² day⁻¹) and sediment (77–224 mg P m⁻² day⁻¹) zones but lower in the epiphytes (1.1–5.2 mg P m⁻² day⁻¹). Total (and/or organic) C (111–311 g C m⁻²), N (9.4–27.2 g N m⁻²), and P (212–1,623 mg P m⁻²) content were the highest in the sediment surface and typically the lowest in the seagrass epiphytes, ranging from 0.6 to 8.7 g C m⁻², 0.02–0.99 g N m⁻², and 0.5–43.5 mg P m⁻². Unlike nutrient content and enzyme activities, bacterial production was highest in the epiphytes (8.0–235.1 mg C m⁻² day⁻¹) and sediment surface (11.5–233.2 mg C m⁻² day⁻¹) and low in the water column (1.6–85.6 mg C m⁻² day⁻¹). At an assumed 50% bacterial growth efficiency, for example, extracellular enzyme hydrolysis could supply 1.8 and 69% of epiphytic and sediment bacteria carbon demand, respectively, while pelagic bacteria could fulfill their carbon demand completely by enzyme-hydrolyzable organic matter. Similarly, previously measured T. testudinum extracellular photosynthetic carbon exudation rates could not satisfy epiphytic and sediment surface bacterial carbon demand, suggesting that epiphytic algae and microphytobenthos might provide usable substrates to support high benthic bacterial production rates. PCA revealed that T. testudinum nutrient content was related positively to epiphytic nutrient content and carbon hydrolase activity in the sediment, but unrelated to pelagic variables. Phytoplankton biomass correlated positively with all pelagic components and sediment aminopeptidase activity but negatively with epiphytic alkaline phosphatase activity. In conclusion, seagrass production and nutrient content was unrelated to pelagic bacteria activity, but did influence extracellular enzyme hydrolysis at the sediment surface and in the epiphytes. This study suggests that seagrass-derived organic matter is of secondary importance in Florida Bay and that bacteria rely primarily on algal/cyanobacteria production. Pelagic bacteria seem coupled to phytoplankton, while the benthic community appears supported by epiphytic and/or microphytobenthos production.     

Distribution,life history,and production of three species of <Emphasis Type="Italic">Neomysis</Emphasis> in Akkeshi-ko estuary,northern Japan

Spatial and seasonal distribution pattern, life history and production of three species of Neomysis (Mysidacea) which commonly occur in northwestern subarctic Pacific coastal waters, were investigated throughout the year in the Akkeshi-ko estuary, northern Japan. The most abundant species Neomysis awatschensis (annual mean density: 179.8 inds. m⁻², biomass: 108.8 mg DW m⁻²) occurred at the inner part of the estuary including low salinity areas with no clear preference for the seagrass bed. The second most abundant Neomysis mirabilis (mean density: 95.8 inds. m⁻², biomass: 90.1 mg DW m⁻²) occurred at relatively saline seagrass site throughout the year. Occurrence of Neomysis czerniawskii in the estuary was limited to the seagrass bed during summer when their population mainly consisted of juveniles, suggesting that this species is a seasonal migrant between the estuary and the marine environment. Both N. awatschensis and N. mirabilis populations were composed of two generation types, a larger sized overwintering and smaller sized spring/summer generations; however, each species had a different reproductive strategy. N. awatschensis was characterized by fast growth to maturity at a smaller size than N. mirabilis with a relatively high fecundity during warm season, suggesting that this species is an r-strategist which can utilize opportunistically a wide variety of habitats. In contrast, the seagrass bed resident N. mirabilis was a K-strategist which matures at a larger size producing fewer but larger offspring. The annual production of N. awatschensis (0.57–0.70 g DW m⁻², mean of the whole estuary) and N. mirabilis (0.58–0.68 g DW m⁻², mean of the seagrass bed) at their respective habitats was comparable. Consequently, species-specific life history and distribution pattern are concluded to allow Neomysis spp. to coexist in the estuary and the high carrying capacity of seagrass bed is suggested to contribute to maintain their high biomass level.     

Productivity of bacteria and microalgae and the effect of grazing by holothurians in sediments on a coral reef flat

Bacterial productivity in sandy sediments on reef flats at Lizard Island, Great Barrier Reef was determined from the rate of incorporation of tritiated thymidine into DNA. The study was conducted during January 1982 and July 1983. A small diurnal increase occurred in sediments having a dense population of microalgae. Bacterial production was 120 to 370 mg C m^-2 d^-1 in summer on reef flats, which was equivalent to 30–40% of primary production by benthic microalgae. In winter, rates of primary production by benthic microalgae and secondary production by bacteria were about one-half to one-fifth of those in summer. There was much variation in production, due to patchiness in the distribution of benthic microbes, especially microalgae. Doubling times for the bacteria in surface sediment were 1 to 2 d in summer and 4 to 16 d in winter on the reef flats. These high productivity values for bacteria indicated that a net input of organic matter to the sediment was needed to support the growth of bacteria. Sediment bacteria thus have a very important role in transforming organic matter on the reef flats. Grazing by Holothuria atra depressed both primary production and bacterial production. It was estimated that these holothurians ate about 10 to 40% of bacterial carbon produced each day in summer, and thus have an important role in the carbon cycle. Harpacticoid copepods were numerically important components of the benthic meiofaunal community and probably had a significant impact on bacterial density as grazers.     

Plankton of the fladen ground during FLEX 76 III. Vertical distribution,population dynamics and production of Calanus finmarchicus (Crustacea: Copepoda)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR), the Longhurst Hardy Plankton Recorder (LHPR) and by our colleagues from other participating Institutes during the Fladen Ground Experiment (FLEX 76) were used to describe the vertical distribution and population dynamics of Calanus finmarchicus (Gunnerus) and to provide estimates of the production and carbon budget of the population from 19 March to 3 June, 1976. Total production of the 19 March to 3 June, 1976. Total production of the nauplii and copepodite stages (including adults), during the exponential growth phase in May, was estimated to be in the range of 0.49 to 0.91 g C m^-2 d^-1 or 29.0 to 55 g dry wt m^-2 (14.5 to 27.8 g C m^-2) for the three successive 10 d periods in May. Two gross growth efficiencies (K ₁) (20 and 34%), together with the lower value of C. finmarchicus production, were used to calculate the gross ingestion levels of algae as 2.45 and 1.44 g C m^-2 d^-1 (73.5 and 43.2 g C m^-2 over the May period). These ingestion levels, together with the algae ingested by other zooplankton species, are greater than the estimated total phytoplankton production of 45.9 g C m^-2 over the FLEX period. A number of factors are discussed which could explain the discrepancies between the production estimates. One suggestion is that the vertical distribution of the development stages of this herbivorous copepod and their diel and ontogenetic migration patterns enable it to efficiently exploit its food source. Data from the FLEX experiment indicated that the depletion of nutrients limited the size of the spring bloom, but that it was the grazing pressure exerted by C. finmarchicus which was responsible for the control and depletion of the phytoplankton in the spring of 1976 in the northern North Sea.JONSDAP Contribution No. 51     

Summer copepod production in subtropical waters adjacent to Australia's North West Cape

Growth and secondary production of pelagic copepods near Australia's North West Cape (21° 49 S, 114° 14 E) were measured during the austral summers of 1997/1998 and 1998/1999. Plankton communities were diverse, and dominated by copepods. To estimate copepod growth rates, we incubated artificial cohorts allocated to four morphotypes, comprising naupliar and copepodite stages of small calanoid and oithonid copepods. Growth rates ranging between 0.11 and 0.83 day^–1 were low, considering the high ambient temperatures (23–28°C). Calanoid nauplii had a mean growth rate of 0.43±0.17 day^-1 (SD) and calanoid copepodites of 0.38±0.13 day^-1. Growth rates of oithonid nauplii and copepodites were marginally less (0.38±0.19 day^–1 and 0.28±0.11 day^–1 respectively). The observed growth rates were suggestive of severe food limitation. Although nauplii vastly outnumbered copepodite and adult copepods, copepodites comprised the most biomass. Copepodites also contributed most to secondary production, although adult egg production was sporadically important. The highest copepod production was recorded on the shelf break (60 mg C m^-2 day^-1). Mean secondary production over both shelf and shelf break stations was 12.6 mg C m^-2 day^-1. Annual copepod secondary production, assuming little seasonality, was estimated as ~ 3.4 g C m^-2 year^-1 (182 kJ m^-2 year^-1).Communicated by G.F. Humphrey, Sydney     

Life cycle of <Emphasis Type="Italic">Pseudocalanus acuspes</Emphasis> Giesbrecht (Copepoda,Calanoida) in the Central Baltic Sea: II. Reproduction,growth and secondary production

The population dynamics of Pseudocalanus acuspes in the Central Baltic Sea were studied from March 2002 to May 2003 on a monthly basis. All stages were present year round with a stage shift from nauplii to older copepodite stages over the course of the year. Biomass, estimated from prosome length, peaked between May and September with maximum recorded values of 594 and 855 mg C m⁻² in May 2002 and 2003, respectively. Differences in biomass between stations up to a factor of 20 were observed especially in April/May and October. Mean egg production rate (EPR) showed a seasonal course and was highest in April 2002 and 2003 with 3.6 and 2.1 eggs f⁻¹ day⁻¹, respectively, corresponding to a mean weight-specific egg production rate (SEPR) of 0.13 and 0.04. Egg production seems to be limited by food from May on. Stage durations determined from moulting experiments turned out to be extremely long. Maximum growth rates based on stage durations of 15–25 days at 4°C in May and July 2003 amounted for 0.03–0.05 day⁻¹ in CI-CIV. Comparing these rates with rates derived from temperature–development relationships for P. acuspes from the literature resulted in five times higher growth rates for the latter case. Secondary production reached values up to 9.1 mg C m⁻² day⁻¹ (method for continuously reproducing populations) and 10.5 mg C m⁻² day⁻¹ (increment summation).     

Phytoplankton exudate release as an energy source for the growth of pelagic bacteria

The release of dissolved organic carbon (DOC) from phytolankton during photosynthesis, and the utilization of this carbon by planktonic bacteria, was studied using ¹⁴CO₂ and selective filtration. Natural sea water samples from a coastal area of the Northern Baltic Sea were incubated in the laboratory for detailed studies, and in situ for estimation of annual dynamics. In a laboratory incubation (at +1°C) the concentration of ¹⁴C-labelled dissolved organic carbon increased for about 2 h and then reached a steady state, representing about 0. 1% of the total DOC. Labelled organic carbon in the phytoplankton and bacterial fractions continued to increase almost linearly. The continuous increase in the bacterial fraction is thought to represent almost instantaneous utilization of the DOC released from the phytoplankton during photosynthesis. As an annual average, in 4 h in situ incubations, about 65% of the labelled organic carbon was found in the phytoplankton fraction (>3 m), about 27% in the bacterial fraction (0.2 to 3 m) and the remaining 8% as DOC (<0.2 m). Large variations in these percentages were recorded. The measured annual primary production was 93 g C m^-2 (March to December), and the estimated bacterial production due to phytoplankton exudates 29 g C m^-2. This represents a release of DOC of about 45% of the corrected annual primary production of 110 g C m^-2 (assuming a bacterial growth efficiency of 0.6).     

Plankton of the fladen ground during FLEX 76 II. Population dynamics and production of Thysanoessa inermis (Crustacea: Euphausiacea)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR) and the Longhurst-Hardy Plankton Recorder (LHPR) during the Fladen Ground Experiment in 1976 (FLEX 76) are used to describe the vertical distribution and population dynamics of Thysanoessa inermis (Krøyer) and to provide estimates of the production and carbon budget of the population from 19th March to 3 June 1976. Spawning occurred in late April and early May, in near synchronisation with the start of the spring bloom of phytoplankton. Eggs, nauplii and calyptopes reached maximum abundance in succession, and furciliae were numerous when sampling ceased in early June. Adults increased in length from a mean of 12.1 mm in mid-March to 17.5 mm in early June and the estimated production was 2.40 mg m^-3 over the 74 d period. Total carbon ingested by the population of T. inermis was estimated to be 10 mg C m^-2 d^-1 in the upper 100m which was only 1.5% of the daily primary production of 0.68 gC m^-2 measured over the FLEX period 26 March to 4 June 1976. The grazing by T. inermis on the phytoplankton population was assumed to have little effect on the control and depletion of the spring phytoplankton bloom during FLEX 77.JOSDAP Contribution No. 50     

Primary productivity and the flux of dissolved organic matter in several marine environments

Primary productivity and the flux of DO¹⁴C, dissolved saccharides (DSAC) and dissolved free primary amines (DFPA) were followed in the Sargasso Sea, Caribbean and upwelling waters of Peru. Average carbon fixation rates were 42.8, 292.8 and 4791.6 mg C m^-2 d^-1, respectively, with nocturnal respiration rates ranging from 9.8–16.3% of gross photosynthesis for the 3 areas. The release of DO¹⁴C, as a percentage of the total carbon fixed in photosynthesis, was non-detectable in the Sargasso Sea, and 3.2 and 4.4% for the Caribbean and Peruvian phytoplankton communities. Few significant changes in DSAC concentrations were recorded over a 36-h incubation period in the Sargasso Sea and Caribbean stations, whereas light-dependent accumulations of DSAC and DFPA were noted in Peruvian stations which were strongly correlated with total phytoplankton productivity. In the Peruvian stations, the average accumulation rate was 234 mg DSAC-C m^-2h^-1 while the average rate of nocturnal decomposition was 141 mg DSAC-C m^-2h^-1; diurnal and nocturnal rates of DFPA accumulation and decomposition were similar (2 mg DFPA-C m^-2h^-1). These data were used to calculate bacterial production in the upwelling waters of Peru. A general discussion of ¹⁴C-technique and routine analytical techniques for DSAC analysis is presented, as DSAC flux exceeded DO¹⁴C flux by 17-fold in coastal Peruvian stations.     

Seasonal growth and productivity ofMacrocystis integrifolia in British Columbia,Canada

The seasonal growth rates and nitrogen and carbon fluxes were estimated for two subtidalMacrocystis integrifolia Bory kelp forests in British Columbia, Canada from changes in population structure through time. Mean relative growth rates of the forests varied from a high of 4.3% d^-1 to a low of-3.6% d^-1. Mean net assimilatioon rates of carbon (a photosynthesis analog) varied from a high of 0.66 g C m^-2 of foliage d^-1 to a low of-0.87 g C m^-2 d^-1. The leaf area index ranged from 0.3 to 11.9. Annual carbon input on a foliage area basis was calculated at 250 g C m^-2 yr^-1. Annual carbon input to the forest was estimated at 1 300 g C m^-2 of ocean bottom yr^-1. The yearly nitrate nitrogen input to the forest was estimated at 60 g N m^-2 of ocean bottom yr^-1. The net ecosystem production varied from-520 to +31 g C m^-2 of ocean bottom yr^-1. The intra-forest, inter-forest and seasonal variabilities of these productivity parameters are discussed.     

Temporal and spatial variation in bacterial production in the water column over a coral reef

Production and doubling times of the bacterial populations in the water around and over the reefs at Lizard Island, Great Barrier Reef were measured during summer and winter, 1982 and 1983. Bacterial productivity, determined from the rate of tritiated thymidine incorporation into DNA, was high over the reef flats and a Thalassia hemprichii sand flat (28 to 58 g Cl^-1 d^-1). Bacterial growth rates increased during the day and fell at night over the reef flats and seagrass bed. Growth rates were slower over the reef front and in open water. Doubling times ranged from about 2 d in the open water to about 3 h over the reef flat in summer. As numbers did not increase, grazing was probably intense on the reef flats. Growth rates were much slower in winter. The main source of organic nutrient used by the bacteria was probably mucus released following photosynthesis in the corals. The cyanobacterium Synechococcus sp. was sometimes very numerous, especially in summer when 2×10⁸ cells l^-1 were recorded in one water mass. The number of bacteria was also very high in summer, with values ranging from 1×10⁹ to 2.5×10⁹l^-1.     

Abundances,growth rates,and production of tropical neritic copepods off Kingston,Jamaica

Weekly samples were collected near Kingston, Jamaica in 27 m vertical hauls, using 200 and 64µm mesh plankton nets, from July 1985 to January 1987. Thirtytwo copepod species were identified; nauplii and all copepodite stages were enumerated. Total copepod abundance ranged from 2.56 to 87.3 × 10⁴ m^–2. The annual abundance cycle was bimodal with peaks in October–November and May–June corresponding to the rainy seasons. Mean annual copepodite biomass was 0.15 g AFDW m^–2 ranging from 0.03 to 0.41 g AFDW m^–2. Mean generation time (from egg to adult) at 28°C was 19.5 d for the common speciesCentropages velificatus, Paracalanus aculeatus, andTemora turbinata. Isochronal development was demonstrated for copepodites ofP. aculeatus andT. turbinata, but not forC. velificatus. Mean daily specific growth rates (G) were 0.63, 0.63, and 0.48 d^–1 forC. velificatus, P. aculeatus, andT. turbinata, respectively. In general, daily specific growth rates decreased in the later copepodite stages. Thus, it is postulated that growth of later stages and egg production may be food limited. Annual copepodite production was estimated as 419 kJ m^–2 yr^–1, while annual exuvial production and naupliar production were 35 and 50 kJ m^–2 yr^–1, respectively. Egg production was estimated as 44% (184 kJ m^–2 yr^–1) of the total copepodite production. Thus, mean total annual copepod production was 688 kJ m^–2 yr^–1. This estimate is within the range of copepod production estimates in coastal temperate regions.     

Light absorption,photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 mol m^-2 s^-1. Growth was lightsaturated at PFDs in excess of 100 mol m^-2 s^-1, with a mean light-saturated growth rate at 23 °C of 1.5×10^-5s^-1 (1.2 d^-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 mol m^-2s^-1. Dark-respiration rates were independent of PFD, averaging 1.7×10^-6 mol O₂ mol^-1 C s^-1 (0.14 mol O₂ mol^-1 C d^-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O₂ mol^-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10^-3 m² mg^-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg^-1 chl a between PFDs of 14 and 200 mol m^-2 s^-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.     

Production primaire pélagique de l'Etang de Berre en 1977 et 1978. Comparaison avec le milieu marin (Méditerranée nord-occidentale)

Measurements of primary production and photosynthetic efficiency were carried out in the brackish lake “Etang de Berre” near Marseilles (France), which is diluted by the Durance River, and in the area of Carry-le-Rouet (Mediterranean Sea) about 25 km off the Rhône River outlet. Primary production (¹⁴C) estimations were made in Etang de Berre from December 1977 to November 1978. The carbon uptake rates ranged between 38 and 1 091 mg C m^-3 d^-1, with an average surface value of 256 mg; water-column carbon-uptake rates ranged between 240 and 2 310 mg C m^-2 d^-1, with an average of 810, representing 290 g C m^-2 per year and 45 000 tons per year of photosynthetized carbon for the whole lake. Gross photosynthetic production measured by the method of Ryther was studied over a 2 yr period. The values obtained from marine water (Carry-le-Rouet) ranged from 23 to 2 337 mg C m^-2 d^-1, with a weighted average of 319, representing about 110 g C m^-2 per year. The values in brackish water (Etang de Berre) ranged from 14 to 1 778 mg C m^-2 d^-1, with a weighted average of 682, representing 250 g C m^-2 per year and 38 400 tons per year of photosynthesized carbon for the whole lake. The values derived from both methods of primary production measurements are approximately similar. Net production (computed from biomass estimations by Utermöhl's method) was compared with gross photosynthetic production. The net production in marine water did not display significant variations: most values were usually near zero. On the other hand, net production in brackish water exhibited a number of clear variations compared with concentrations of gross photosynthetic production during the whole 2 yr period. This large difference between estimations of gross and net production may be due to grazing, which is high in Etang de Berre, but slower and more constant in seawater. The ratios of primary production: chlorophyll a and gross photosynthetic production: biomass were also studied. In Etang de Berre, the former ratio ranges between 0.57 and 3.75, with an average of 1.44; this is similar to previously reported values. The ratio gross production: biomass in Etang de Berre varies between 0.3 and 4.2, with an average of 1.27, also confirming previous data. The very high values calculated for marine waters in the present study may result from an under-estimation of biomass.     

Production of tropical copepods in Kingston Harbour, Jamaica: the importance of small species

The copepod community observed during an 18-month period at the mouth of eutrophic Kingston Harbour, Jamaica, was dominated by small species of Parvocalanus, Temora, Oithona, and Corycaeus. Mean copepod biomass was 22.1 mg AFDW m⁻³ (331 mg m⁻²). Annual production was 1679 kJ m⁻², partitioned as 174 kJ m⁻² naupliar, 936 kJ m⁻² copepodite, 475 kJ m⁻² egg and 93 kJ m⁻² exuvial production. All nauplii, most copepodites and many adults, equivalent to half of the biomass and production, were missed by a standard 200-μm plankton net, emphasizing the importance of nauplii and small species in secondary production estimates. The evidence suggests that growth rates and production are generally not food limited, and we speculate that size-selective predation shapes the structure of the harbour community. Biomass and production are higher than previous estimates for tropical coastal waters, but comparable to other eutrophic tropical embayments and many productive temperate ecosystems. Far from being regions of low productivity, tropical zooplankton communities may have significant production and deserve greater research attention than they currently receive. Received: 19 September 1997 / Accepted: 21 October 1997     

Thalassia testudinum productivity and grazing by green turtles in a highly disturbed seagrass bed

There has been an historical decline in the seagrass beds in Maho and Francis Bays, St. John, U.S. Virgin Islands: presently (1986) there are only five small seagrass beds in shallows water. These seagrass beds are highly disturbed by heavy boat usage and are intensively grazed by the green turtle Chelonia mydas L. Fifteen to 50 boats anchor each night in the bays: anchor scars cause a loss of up to 6.5 m² d^-1 or 1.8% yr^-1 of the seagrass beds. Seagrasses regrew into such scars only minimally within a period of 7 mo. The size of the green turtle population was estimated at 50 subadults and their feeding behavior was determined by direct observation and radiotelemetry. The behavior of the green turtles differed from other observations published on the species. Here, the turtles grazed all available Thalassia testudinum, their preferred seagrass food, rather than creating discrete grazing scars, and spent all their waking hours (9 h per day) feeding. Areal productivity of T. testudinum leaves (33 to 97 mg dry wt m^-2d^-1) in the bays was at least an order of magnitude lower than published values or than the productivity of another, lessdisturbed seagrass bed on St. John, despite having comparable leaf-shoot density. Leaf shoots were stunted, fragile, achlorotic, and had only two leaves as opposed to the five leaves per shoot more typically seen. The green turtle population was near the estimated carrying capacity of T. testudinum, based on the standing crop and productivity of T. testudinum and the grazing rate of the turtles. The effect of disturbance of T. testudinum from boats and turtles was assessed by excluding these with emergent fences. Within 3 mo of protection, the areal and shoot-specific productivity of T. testudinum leaves as well as leaf size increased significantly compared to unprotected areas. Conservation efforts are recommended in Maho Bays and Francis because seagrass productivity is low, disturbance rates are higher than recovery rates, the turtles cannot increase further their feeding rate in order to compensate for such factors, and there are few alternate sources of T. testudinum on the north shore of St. John.Contribution No. 175 from West Indies Laboratory, Teague Bay, Christiansted, St. Croix, U.S. Virgin Islands 00820, USA     

Population biology of hyperbenthic crustaceans in a cold water environment (Conception Bay,Newfoundland). II.<Emphasis Type="Italic"> Acanthostepheia malmgreni</Emphasis> (Amphipoda)

Seasonal population dynamics of the gammarid Acanthostepheia malmgreni Goës in Conception Bay, Newfoundland, were examined from October 1998 to November 2000. This species exhibited a 2.5-year life span, with the reproductive cycle correlating with seasonal phytoplankton flux. Females were semelparous and died following a 5-month brooding period and the subsequent release of juveniles in April and May. The biennial life cycle of this population should result in the presence of two cohorts in the hyperbenthos at any given time. However, the cohorts alternated in strength from year to year, which affected annual density, biomass and production during the study period. Densities were 64±87 ind. per 100 m³ in 1999 and 491±492 ind. per 100 m³ (mean±SD) in 2000. Secondary production was estimated at 18–44 mg C m^–2 in 1999 and 180–311 mg C m^–2 in 2000. The annual P/B ratios were 0.89 and 2.27 in 1999 and 2000, respectively. Growth varied both among and within cohorts, with different life-history stages exhibiting variable growth rates ranging from 0 to 12 mg dry mass month^–1.Communicated by J.P. Grassle, New Brunswick     

Net productivity of two cohorts of Chordaria flagelliformis (Phaeophyta) in Nova Scotia,Canada

Productivities of two cohorts of Chordaria flagelliformis (O. F. Müll.) C. Ag. were estimated from measured changes in biomass and survivorship over time. Maximum productivity during the summer growing season was 2.6 g C m^-2 d^-1. Although this figure is relatively high, the short growing season results in an annual production of only 89 g C m^-2. The significance of primary production by C. flagelliformis lies in its seasonal timing. During the summer growth period, 50% of production was recycled directly by detrital material. During the same time period, productivity and biomass losses of other seaweeds are at their lowest.     

Production organique primaire dans un milieu saumâtre eutrophe (Etang de Berre). Effets d'une forte dilution (dérivation des eaux de la Durance)

¹⁴C primary production measurements were made over a period of 5 years (1965–1969, inclusive) in the brackish lake Etang de Berre, near Marseilles (France). The diversion of the River Durance into the Etang de Berre took place during this period (March 1966) and introduced an important modification into the organic production ecosystem, mainly through increased and variable freshening, accompanied by substantial nutrient input. The seasonal distribution of production rates displayed 3 bloom periods: the first (short and slight) in spring, the second (the most important as regards intensity and duration) in summer, and the third in autumn (October). Before the diversion of the river in 1965, the carbon-uptake rates in the lake ranged between 25 mg/m²/day in winter and 800 mg/m²/day in summer-autumn, the mean value for the year being 150 g C/m², which represents 2.5×10⁴ tons of photosynthesized carbon for the whole lake. After the diversion, more than 3000 mg C/m² day were measured; for 1968, the inclusive uptake rate was 384 g C/m², representing 6×10⁴ tons of synthesized carbon for the whole lake. Nevertheless, noticeable variations occurred from one year to another. From the annual nutrient input of phosphate to the Etang de Berre through the inflow of Durance waters, the quantity of potentially synthesizable elements has been calculated, in terms of carbon, according to the normal P:C ratio of organic substances; this quantity is called R. The difference between measured production, P, and R gives a measure of the regenerated production. This portion of production represented about 80% of the total production before 1968 but only 16% in 1969, a year of maximum fresh-water inflow. This phenomenon could be due to modifications of the ecophysiology of the phytoplankton resulting from the considerable freshening. With increasing nutrient load, eutrophication first occurs, then still greater dilution results in inhibition of production.     

Rates of ammonium turnover and the role of amino-acid deamination in seagrass (Zostera capricorni) beds of Moreton Bay,Australia

Samples of sediments from Australian seagrass (Zostera capricorni Aschers.) beds were taken in June to August 1983 (for¹⁵N experiments) and November 1982 to January 1983 (¹⁴N experiments). The ammonium pool turned-over every 0.4 to 0.8 d, as determined with a¹⁵N isotope-dilution technique. The ammonium pool in subtidal bare areas turned-over two to three times more slowly than in adjacent seagrass beds. Gross rates ofin situ ammonium regeneration equalled those of utilization, and ranged from 0.04 to 0.35 mol cm^-3 d^-1, or from 50 to 490 mg N m^-2 d^-1 over the upper 10 cm of the sediment. The potential rate of glycine utilization, measured with a large excess of glycine added to anaerobic incubations, ranged from 0.21 to 0.39mol cm^-3 d^-1, butin situ rates were probably much lower. Between 35 and 65% of added¹⁵N-glycine was deaminated over 12 h, and the remainder was most likely assimilated by microbes. Evidence for the seagrasses taking up glycine was equivocal, owing to the rapid deamination of the amino acid and the likelihood that they assimilated the labelled ammonium produced from the glycine.