Patricia Kremer

The contribution of microorganisms to particulate carbon and nitrogen in surface waters of the Sargasso Sea near Bermuda

Abstract Seawater samples were collected from the euphotic zone of the Sargasso Sea near Bermuda in August of 1989 and March–April of 1990. Microbial population abundances, chlorophyll concentration, particulate carbon and particulate nitrogen were measured. Calculations were performed to establish the relative and absolute importance of the various microbial assemblages. The choice of conversion factors (g C and N cell −1 , or g C and N μm −3 ) for the microbial populations dramatically affected the estimation of “living” and “detrital” particulate material in the samples, and the relative importance of the various microbial groups. Averaged over all samples on either of the two cruises, microbial biomass constituted a greater proportion of the total particulate carbon and nitrogen during March–April (55% and 63%, respectively), than during August (≈24% and 30%, respectively) using “constrained” conversion factors that were derived. Accordingly, detrital material constituted the bulk of the particulate material during August, but was similar to the amount of microbial biomass during March–April. The bacterial assemblage constituted the largest single pool of microbial carbon (35%) and nitrogen (45%) in the water, and a significant fraction of the total particulate carbon (≈10–20%) and nitrogen (≈15–30%). Phototrophic nanoplankton (microalgae 2–20 μm in size) were second in overall biomass, and often dominated the microbial biomass in the deep chlorophyll maxima that were present during both cruises. The results temper recent assertions concerning the overwhelming importance of bacterial biomass in the oligotrophic Sargasso Sea but still support a major role for these microorganisms in the open ocean as repositories for carbon and nutrients.

Effect of food availability on the metabolism of the ctenophore Mnemiopsis mccradyi

Measurements of respiration and excretion for the ctenophore Mnemiopsis mccradyi Mayer at 21°C at 4 concentrations of food (copepods: primarily Acartia tonsa) showed a marked effect of food availability on the metabolic rate. Starved ctenophores respired at a rate of 6.0 μg-at O2 h-1 g-1 dry wt, and excreted at 0.42 μg-at N−NH4 h-1 g-1 dry wt. Ctenophores fed at 500 prey l-1 respired at a rate of 26.4 μg-at O2 h-1 g-1 dry wt and excreted at a rate of 1.8 μg-at N−NH4 h-1 g-1 dry wt. Freshly collected ctenophores from Biscayne Bay, Florida, during November 1979, had intermediate metabolic rates. The atomic O:N ratio (based on ammonium alone) averaged 12:1 and was fairly constant for all conditions. The release of organic nitrogen was approximately equal to ammonium excretion, and urea was about 10% of the total. The elemental composition of carbon and nitrogen as a percentage of the dry weight varied directly with food availability. Total nitrogen turnover ranged from 8% per day for starved ctenophores, to 19% per day for the highest food concentration. Carbon turnover rates were higher. M. mccradyi exhibited a clear shift upwards in their excretion rate in response to the time they had been feeding, reaching a maximum after about 2h. When ctenophores were removed from food, the metabolic rate declined exponentially with time, the most rapid change occurring in the first few hours of starvation. Comparisons of carefully measured metabolic rates for freshly collected ctenophores with experimental results such as these may help to establish the nutritional state and extent of food limitation in situ.

Spatial and temporal changes in the partitioning of organic carbon in the plankton community of the Sargasso Sea off Bermuda

The vertical distribution of plankton (bacteria, nanozooplankton, microzooplankton, mesozooplankton, macrozooplankton and salps) biomass in the photic zone near the JGOFS time series station off Bermuda was examined during 2–3 week periods in August 1989 and in March/April 1990. The amount of phytoplankton carbon in the photic zone was lower in August as compared to March/April (398 and 912 mg C m−2, respectively). Total heterotrophic biomass in the photic zone was also lower in August as compared to March/April (1106 and 1795 mg C m−2, respectively). Taken together, bacteria and nanozooplankton constituted approximately 70% of the total heterotrophic carbon in the photic zone on both cruises. Considering their high weightspecific carbon demand relative to micro-, meso-, and macrozooplankton, it is clear that most of the carbon in the surface waters of the Sargasso Sea near Bermuda cycles through bacteria and flagellates—the “microbial loop”. However, both seasonal (August vs. March/April) and withincruise variations in the vertical flux of organic material were related to the biomass of macrozooplankton. Macrozooplankton biomass was lower in August than March/April (93 and 267 Mg C m−2, respectively). There was more non-living carbon (detritus) than living carbon in the photic zone during the August cruise (70% of total organic matter) but about equal amounts of detritus and living carbon in March/April.

Metabolism of epipelagic tropical ctenophores

Measurements of respiration and excretion at 25°C were made for five species of ctenophores collected during five cruises to the Bahamas (1982–1984). The mean element-specific respiration and ammonium excretion rates of freshly collected specimens of all species ranged from 4 to 16% d-1, the mean atomic O:N ratios were 10 to 16, and ammonium averaged 60 to 90% of the total dissolved nitrogen excreted. For adult ctenophores, the carbon content ranged from 0.6% carbon (as percent of dry weight) for Bolinopsis vitrea to 3.7% carbon for Beroë ovata. There was a marked increase in the organic content (% carbon of dry weight) of small Bolinopsis vitrea with tentacles compared to fully lobate adults. B. vitrea had increasingly higher metabolic rates when held at food concentrations up to 100 copepods 1-1 (about 250 μg C 1-1). The overall range between starved and well-fed B. vitrea was about two times for respiration and a factor of three for ammonium excretion. B. vitrea decreased from well-fed to a starved metabolic rate in about a day after removal from food. The metabolic rate of Eurhamphaea vexilligera was not measurably affected by short-term starvation or feeding (maximum 25 copepods 1-1). In feeding experiments, E. vexilligera of 20 to 56 mm length fed at rates equivalent to clearance rates of 250 to 1 800 ml h-1.

Respiration and excretion by oceanic salps

Rates of oxygen consumption and ammonium nitrogen excretion were measured on the solitary and/or aggregate generations of ten species of oceanic salps collected by SCUBA divers during cruises in the Atlantic Ocean (1982–1985). Species that were visibly more active had higher metabolic rates than did less active species. Rates were 1.5 to 2 times lower and O:N ratios were lower when salps were held before incubation than when incubation began at the time of collection. Respiration rate showed a better relationship to length than to weight, suggesting that metabolic activity may be connected mainly with swimming. O:N ratios were between 13 and 28 for most species and generations, but higher and more variable in Pegea spp. Exretion of urea was low or undetectable. Rates of metabolic demand (turnover) ranged from 9.7 to 99% body carbon d-1 and 6.4 to 55.6% body nitrogen d-1.

Chemical composition, metabolic rates and feeding behavior of the midwater ctenophore Bathocyroe fosteri

Individuals of the midwater ctenophore Bathocyroe fosteri (0.01 to 1.6 g dry weight, DW) were collected from Bahamian waters by the submersible “Johnson-Sea-Link” during May and September/October 1983 and October/November 1984 from 530 to 700 m depth. Metabolic rates were measured and showed oxygen consumption to be in the range of 0.01 to 0.18 mg O2 g-1 DW h-1 at temperatures ranging from 9° to 12°C. Ammonium excretion (0.01 to 0.14 μg-at N g-1 DW h-1) was typically low. Energy expenditures estimated from respiration data (ca. 7% body C d-1) indicated that one to three midwater crustacean prey (ca. 150 μg C d-1) could provide the daily maintenance ration required by a 40 mm ctenophore. These metabolic characteristics complemented in situ observations of poor locomotor ability and passive feeding behavior.

Transmission of symbiotic dinoflagellates through the sexual cycle of the host scyphozoan Linuche unguiculata

Intracellular symbiotic dinoflagellates are associated with the tropical scyphozoan Linuche unguiculata (Swartz, 1788) throughout all stages of the hosts life cycle. During sexual reproduction, eggs are released in mucus strands that contain symbiotic dinoflagellates. Fertilization and development take place externally in the water column. Epifluorescence and transmission electron microscopy showed that unfertilized eggs did not contain intracellular algae, but that infection of the developing embryo was generally successful by the 128-cell stage (≃10 h after fertilization at 23° C). However, experiments with artificially provided Cellufluor-labeled algae demonstrated that older embryos and planulae could be infected by algae through at least 24 h post-fertilization, indicating that the L. unguiculata symbiosis represents a “semi-closed” system. This novel mode of symbiont acquisition results in most sexually-produced offspring becoming infected with maternally-transmitted algae during early development, but allows for acquisition of non-maternally-provided algae later in development. Most of the algal symbionts during the early stages of embryonic and larval development are located within ectodermal cells. This is in contrast to the other life-cycle stages of L. unguiculata (i.e., scyphistoma, medusa, ephyra), where symbionts are found within the gastrodermis of the host.

A three-trophic level estuarine model: Synergism of two mechanistic simulations

Abstract Few numerical simulations have attempted to include a high degree of biological detail for several trophic levels. Typically, in planktonic ecosystem models, if the dynamics of nutrients, phytoplankton and herbivorous zooplankton are formulated with ecological complexity, then carnivores are ignored, forced or modeled in an extremely simplified manner. Extensive mechanistic detail for important carnivores is difficult to represent because reliable and relevant ecological data are rarely available for appropriate species and local populations. Further, the wide temporal and spatial differences between life histories of lower plankton and carnivores may be technically difficult to model. In Narragansett Bay, Rhode Island, the ctenophore Mnemiopsis leidyi is an important carnivore to which these objections do not apply. A detailed carbon-based simulation model of this population of ctenophores was developed independently from an ecosystem model of Narragansett Bay which included detailed interactions between phytoplankton, primarily herbivorous zooplankton and nutrients. The interfacing of these two models without changing any of the formulations or values of the coefficients provided a test of the commonly used practice of forcing certain components. Both models were originally constructed with the biomass of a critical compartment forced according to observed data; in the plankton model, ctenophores were forced, and in the ctenophore model, zooplankton were forced. Predicted biomasses for zooplankton and ctenophores in the combined model were similar to the results of the two parent models, but improved relative to the actual field observations. From the findings it appears that the strategy of forcing is valid provided the forced patterns are appropriate and reasonable.

Ecological simulation model of Los Angeles Harbor

A quasi-steady state numerical ecosystem model was designed to help evaluate the potential impact of various scenarios of effluent treatment and of a landfill on the distribution of phytoplankton and inorganic nutrients in Los Angeles and Long Beach harbors Formulations included (a) tidal circulation, (b) phytoplankton growth and oxygen production as a function of temperature, light, and nutrients, (c) grazing by zooplankton, (d) respiration and nutrient regeneration by the benthos, (e) biochemical oxidation of organics, and (f) nitrification Phytoplankton nitrogen, ammonium, nitrate, and oxygen were the state variables, which were simulated with diel and spatial variability for a range of seasonal conditions. Physical circulation was indicated to be a primary factor governing the distribution of state variables, and the landfill resulted in significant alterations.Simulated phytoplankton stocks approximated the upper range of reported concentrations, indicating a satisfactory prediction of bloom conditions. The model indicated that while light may usually regulate maximum phytoplankton levels, under bloom conditions nutrient limitation may also be important Most of the outer Los Angeles Harbor was affected by the effluent, as shown by comparison to the case with zero input Simulations for secondary versus primary treatment converged a short distance from the outfall in response to high BOD oxidation rates. In general, total phytoplankton crop was not greatly affected by the change from primary to secondary treatment, and predation on phytoplankton was small