Phytoplankton carbon dynamics during a winter-spring diatom bloom in an enclosed marine ecosystem: primary production,biomass and loss rates

Phytoplankton production, standing crop, and loss processes (respiration, sedimentation, grazing by zooplankton, and excretion) were measured on a daily basis during the growth, dormancy and decline of a winter-spring diatom bloom in a large-scale (13 m³) marine mesocosm in 1987. Carbonspecific rates of production and biomass change were highly correlated whereas production and loss rates were unrelated over the experimental period when the significant changes in algal biomass characteristic of phytoplankton blooms were occurring. The observed decline in diatom growth rates was caused by nutrient limitation. Daily phytoplankton production rates calculated from the phytoplankton continuity equation were in excellent agreement with rates independently determined using standard ¹⁴C techniques. A carbon budget for the winter bloom indicated that 82.4% of the net daytime primary production was accounted for by measured loss processes, 1.3% was present as standing crop at the end of the experiment, and 16.3% was unexplained. Losses via sedimentation (44.8%) and nighttime phytoplankton respiration (24.1%) predominated, while losses due to zooplankton grazing (10.7%) and nighttime phytoplankton excretion (2.8%) were of lesser importance. A model simulating daily phytoplankton biomass was developed to demonstrate the relative importance of the individual loss processes.     

Seasonal variations in microzooplankton grazing in the region of the Subtropical Convergence

Microzooplankton grazing and community structure were investigated in the region of the Subtropical Convergence (STC) during three cruises of the South African Antarctic Marine Ecosystem Study (SAAMES) in austral summer (January/February 1993; December 1994/January 1995) and winter (June/July 1993). Chlorophyll a concentrations were consistently dominated by the <20 m size fraction during all three cruises, while the contribution of the microphytoplankton (>20 m) to total chlorophyll a concentrations varied considerably between cruises. Microzooplankton communities were numerically dominated by protozoans comprising ciliates (aloricates and tintinnids) and dinoflagellates. Instantaneous growth coefficients of phytoplankton in the vicinity of the STC showed no seasonal trends. However, marked seasonal differences were observed in the size structure of the phytoplankton. The grazing impact of microzooplankton was highest when the <20 m chlorophyll fraction contributed >95% of the total. Under these conditions, the instantaneous grazing rates ranged between 0.15 and 0.66 d^-1. These correspond to daily losses of 14 to 48% of the inntial standing stock and between 45 and 81% of the potential primary production. At stations where microphytoplankton contributed significantly (-20%) to total chlorophyll concentrations, the grazing coefficients were lower, ranging between 0 and 0.53 d^-1. This corresponds to a loss of <41% of the initial standing stock, or between 0 and 56% of the potential production. Our data suggest that microzooplankton represent the main grazing sink for production when the <20 m chlorophyll size-class dominates total chlorophyll. These facts suggest that the efficiency of the biological pump may vary over time.     

In situ grazing rate of the copepod population in the western subarctic North Pacific during spring

Time series sampling with a multi-layer plankton sampler was carried out in the western subarctic North Pacific during spring 1991. Neocalanus cristatus, N. flemingeri and Eucalanus bungii dominated and accounted for 88.5% of the copepod population in volume. Neocalanus spp. were distributed in the upper mixed layer, while E. bungii was mainly distributed between 120 and 300 m throughout the day and night. In contrast, Metridia pacifica, Pleuromamma scutullata and Gaetanus simplex showed clear diel vertical migration. Grazing activities were estimated simultaneously by gut fluorescence. Nocturnal grazing was observed for diel migrating species. Neocalanus spp. did not have a diel feeding rhythm and had relatively low gut fluorescence. E. bungii was considered to be dormant during the observation period. The estimated grazing rate of the copepod population on phytoplankton was 1.4 to 2.0% of the primary production while the metabolic requirement was 8.3 to 12.4% of the primary production. These facts suggest that the copepod population was unimportant as primary consumers and that microzooplankton plays a much more important role in sustaining low standing stock of phytoplankton and a high nutrient concentration in the western subarctic Pacific Ocean.     

Description and use of an improved method for determining estuarine zooplankton grazing rates on phytoplankton

A method of rapidly determining zooplankton grazing rates on natural mixed phytoplankton populations using ¹⁴C is described. The method simplifies the design of grazing experiments as the grazing time can be kept short enough to prevent recycling of the isotope, and growth of the phytoplankton substrate. Very high specific activity, ¹⁴C-labelled phytoplankton concentrated either by centrifugation or sieving, may be used either as the sole grazing substrate, or as a tracer in natural mixed phytoplankton. Zooplankton, confined in glass jars at either ambient, or higher than ambient concentrations, are permitted to feed on the phytoplankton for periods of 30 min and 2 h, and are then separated by sieving. The zooplankton community grazing rate, or, if the samples are sorted into species, the individual species grazing rates, can be determined after scintillation counting of the zooplankton. The rate of appearance of ¹⁴C-labelled phytoplankton in the zooplankton is an estimate of the grazing rate, and the slope of the line joining the grazing rates at various phytoplankton concentrations gives an estimate of the grazing rate constant for the zooplankton population. The method provides a quick way of obtaining both zooplankton population, and individual species grazing rates on natural mixed phytoplankton. In two experiments, labelled phytoplankton was used as the sole grazing substrate in concentrations ranging between 0.4 and 5 times ambient levels. Grazing rate constants, for net-caught zooplankton concentrated to 46 times (Experiment 1) and 28 times (Experiment, 2) ambient estuarine levels were-0.14and-0.12 of the phytoplankton standing stock per day, respectively. There was a linear increase in the amount of phytoplankton grazed with an increase in phytoplankton concentration up to four times ambient phytoplankton levels. When tracer amounts of labelled phytoplankton were added to samples containing both phytoplankton and zooplankton at ambient concentrations the grazing rate constants were-0.28 and-0.42 of the phytoplankton standing stock per day. We conclude that zooplankton grazing was the major control factor of phytoplankton population size during October–November 1975 in South West Arm, Port Hacking, near Sydney, Australia.     

The impact of grazing by microzooplankton in northern Hiroshima Bay,the Seto Inland Seam,Japan

The abundance of microzooplankton and their grazing impact on phytoplankton were studied using the dilution technique from May 1990 to November 1991 in northern Hiroshima Bay, a typical eutrophic area in the Seto Inland Sea. Microzooplankton, dominated in number by tintinnid ciliates, were abundant from June to September when chlorophyll-a concentrations were high. Maximum density of microzooplankton ranged from 3.8×10³ to 25.4×10³ ind l^-1. During the period of investigation, mean microzooplankton density and mean chlorophyll-a concentration of the <20-m fraction increased toward the inner region of the bay. The microzooplankton grazing on phytoplankton increased from summer to early autumn, and decreased from late autumn to winter. At an offshore station, the annual means of the daily grazing loss for total chlorophyll-a and the chlorophyll-a of the <20-m fraction were 12 and 15% of the initial standing stock, respectively. At an estuarine station, the microzooplankton grazed 19 and 29% of the total and <20-m initial standing stock, respectively. The quantity of grazed chlorophyll-a correlated positively and linearly with the potential production of chlorophyll-a at both stations. The quantity of chlorophyll-a grazed by microzooplankton and the potential production of chlorophyll-a were nearly equivalent in the <20-m fraction at the estuarine station. This suggests that the microzooplankton assemblage was able to consume almost all the nanoplankton newly produced in the eutrophic estuary.     

Behavior of a simple plankton model with food-level acclimation by herbivores

The acclimation of herbivores to variation in their phytoplankton food source was expressed mathematically and its effect on phytoplankton, herbivore and nutrient cycles explored with a plankton model. The grazing formulation is a modified version of the function experimentally determined by Mayzaud and Poulet. Their function differs from the traditional Ivlev expression for herbivore grazing in that there is no asymptotic limit to the grazing rate. The steady-state solutions of the phytoplankton-herbivorenutrient model were similar with the two grazing formulations, but the time-dependent behaviour of the two models differed markedly. The model with Ivlev grazing showed oscillations when the grazing pressure was high. The model with acclimated herbivore grazing showed only small, highly damped oscillations as it approached steady state. The latter is more similar to the evolution of plankton trophic levels observed in controlled ecosystem experiments.     

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000     

The role of ciliates,heterotrophic dinoflagellates and copepods in structuring spring plankton communities at Helgoland Roads,North Sea

Mesocosm experiments coupled with dilution grazing experiments were carried out during the phytoplankton spring bloom 2009. The interactions between phytoplankton, microzooplankton and copepods were investigated using natural plankton communities obtained from Helgoland Roads (54°11.3′N; 7°54.0′E), North Sea. In the absence of mesozooplankton grazers, the microzooplankton rapidly responded to different prey availabilities; this was most pronounced for ciliates such as strombidiids and strobilids. The occurrence of ciliates was strongly dependent on specific prey and abrupt losses in their relative importance with the disappearance of their prey were observed. Thecate and athecate dinoflagellates had a broader food spectrum and slower reaction times compared with ciliates. In general, high microzooplankton potential grazing impacts with an average consumption of 120% of the phytoplankton production (P _p ) were measured. Thus, the decline in phytoplankton biomass could be mainly attributed to an intense grazing by microzooplankton. Copepods were less important phytoplankton grazers consuming on average only 47% of P _p . Microzooplankton in turn contributed a substantial part to the copepods’ diets especially with decreasing quality of phytoplankton food due to nutrient limitation over the course of the bloom. Copepod grazing rates exceeded microzooplankton growth, suggesting their strong top-down control potential on microzooplankton in the field. Selective grazing by microzooplankton was an important factor for stabilising a bloom of less-preferred diatom species in our mesocosms with specific species (Thalassiosira spp., Rhizosolenia spp. and Chaetoceros spp.) dominating the bloom. This study demonstrates the importance of microzooplankton grazers for structuring and controlling phytoplankton spring blooms in temperate waters and the important role of copepods as top-down regulators of microzooplankton.     

Dependence of phytoplankton production on rhythm and rate of elimination

This paper investigates the dependence of phytoplankton production upon rhythm and rate of zooplankton grazing and presents a mathematical model for calculating the most important parameters. Both uniform and non-uniform grazing are described mathematically. Non-uniform grazing, expressed by a sinusoidal curve, is usually found in bathyplanktonic ecosystems with migratory consumers. Phytoplankton production depends on the time of grazing; the nearer grazing occurs toward nightfall, the higher is the phytoplankton production. In order to calculate phytoplankton productivity and the amount of food consumed by the zooplankton, experimental data on generation time of phytoplankters, their mortality rates, initial and final standing stocks, and information on diurnal grazing rhythms must be available. If the distribution of grazing rates is sinusoidal and mortality rate constant, the equations presented allow the calculation of phytoplankton productivity with an error of about 6%.     

Microzooplankton herbivory on the Grand Bank (Newfoundland,Canada): A seasonal study

Grazing impact of microzooplankton on phytoplankton was investigated on the Grand Bank, Newfoundland, Canada, in April, July and October 1984, using a seawater dilution method. In April a large proportion of chlorophylla was in the microplankton size fraction (> 20µm) while in mid-summer and fall most was in the nanoplankton size fraction (< 20µm). Diatoms were the dominant phytoplankters in April, while undetermined flagellates and coccolithophores were abundant in other seasons. Major grazers were oligotrichous ciliates in all seasons. Instantaneous grazing rates on nanophytoplankton, as measured by changes in chlorophylla, varied from 0.12 to 0.43 d^–1 and those on microphytoplankton from 0.19 to 0.68 d^–1. Grazing rates did not change over 24 and 48 h intervals. This level of grazing corresponded to a daily loss of about 20 and 30% of standing stock of chlorophylla and about 50 and 70% loss of potential production in the two size fractions respectively. Taxon-specific grazing rates, calculated from microscopic enumeration, showed that small diatoms were grazed heavily, and their growth was controlled by grazing in late spring. In late summer and fall, undetermined flagellates and coccolithophores were also grazed at high rates but their growth rates were higher than the grazing rates, and therefore, were not controlled by microzooplankton. In general, microzooplankton grazed on whatever appropriate sized food was dominant in the experimental water. Their potential ability to control the growth of certain food species may be one of the causes determining the species composition of phytoplankton communities.     

Do plant density, nutrient availability, and herbivore grazing interact to affect phlorotannin plasticity in the brown seaweed Ascophyllum nodosum

Plants have different strategies to cope with herbivory, including induction of chemical defences and compensatory growth. The most favourable strategy for an individual plant may depend on the density at which the plants are growing and on the availability of nutrients, but this has not been tested previously for marine plant–herbivore interactions. We investigated the separate and interactive effects of plant density, nutrient availability, and herbivore grazing on the phlorotannin (polyphenolic) production in the brown seaweed Ascophyllum nodosum. Seaweed plants grown at low or high densities were exposed either to nutrient enrichment, herbivorous littorinid gastropods (Littorina obtusata), or a combination of nutrients and herbivores in an outdoor mesocosm experiment for 2 weeks. Seaweeds grown at a low density tended to have higher tissue nitrogen content compared to plants grown at a high density when exposed to elevated nutrient levels, indicating that there was a density dependent competition for nitrogen. Herbivore grazing induced a higher phlorotannin content in plants grown under ambient, but not enriched, nutrient levels, indicting either that phlorotannin plasticity is more costly when nutrients are abundant or that plants responded to herbivory by compensatory growth. However, there were no significant interactive or main effects of plant density on the seaweed phlorotannin content. The results indicate that plants in both high and low densities induce chemical defence, and that eutrophication may have indirect effects on marine plant–herbivore interactions through alterations of plant chemical defence allocation.     

Phytoplankton photosynthetic activity and growth rates in the NW Adriatic Sea

Taxonomic composition, biomass, primary production and growth rates of the phytoplankton community were studied in two stations in the NW Adriatic Sea on a seasonal basis, in areas characterized by differing hydrological and trophic conditions. The main differences between the two stations were quantitative rather than qualitative, most phytoplankton species being common to both stations. The effects of differing nutrient concentrations and plume spreading were evident. Biomass and primary production rates were significantly higher in the coastal station (S1), and the phytoplankton distribution in the water column was markedly stratified in S1 and more even in the offshore station (S3). However, chlorophyll a specific production, potential growth rate and production efficiencies were very similar in both stations, even when phosphorus concentrations were limiting. A discrepancy between potential and actual growth rate was observed: as a feature common to both stations, comparisons between potential and actual growth rates revealed that little carbon produced by phytoplankton accumulated as algal biomass; therefore, very high loss rates were estimated.     

Microzooplankton grazing and selectivity of phytoplankton in coastal waters

Microzooplankton grazing activity in the Celtic Sea and Carmarthen Bay in summer 1983 and autumn 1984 was investigated by applying a dilution technique to high-performance liquid chromatographic (HPLC) analysis of photosynthetic pigments in phytoplankton present within natural microplankton communities. Specific grazing rates on phytoplankton, as measured by the utilisation of chlorophyll a, were high and varied seasonally. In surface waters during the autumn, grazing varied between 0.4 d^-1 in the bay and 1.0 d^-1 in the Celtic Sea, indicating that 30 and 65% of the algal standing stocks, respectively, were grazed daily. Grazing rates by microzooplankton within the thermocline in summer suggest that 13 to 42% of the crop was grazed each day. Microzooplankton showed selection for algae containing chlorophyll b, in spite of a predominance of chlorophyll c within the phytoplankton community. Changes in taxon-specific carotenoids indicated strong selection for peridinin, lutein and alloxanthin and selection against fucoxanthin and diadinoxanthin. This indicates a trophic preference by microzooplankton for dinoflagellates, cryptophytes, chlorophytes and prasinophytes and selection against diatoms, even when the latter group forms the largest crop within the phytoplankton. Interestingly, those algal taxa preferentially grazed also showed the highest specific growth-rates, suggesting a dynamic feed-back between microzooplankton and phytoplankton. Conversion of grazing rates on each pigment into chlorophyll a equivalents suggests firstly, that in only one experiment could all the grazed chlorophyll a be accounted for by the attrition of other chlorophylls and carotenoids, and secondly that in spite of negative selection, a greater mass of diatoms could be grazed by microzooplankton than any other algal taxon. The former may be due either to a fundamental difference in the break-down rates of chlorophyll a compared to other pigments, or to cyanobacteria forming a significant food source for microzooplankton. In either case, chlorophyll a is considered to be a good measure of grazing activity by microzooplankton.     

Variability and its effect on the 24h chlorophyll budget of a small marine basin

In an intensive study (lasting 25 h) of the production, export and grazing of phytoplankton in a small marine basin, it was found that 58% of the production (11% of the total standing stock) was lost by exchange with the sea and 34% was consumed by grazing of zooplankton. The measured production of phytoplankton could be balanced, to within a few percent, against grazing, export, and a small, measured, net change in the total standing stock of the basin. Large variations were observed in concentrations of chlorophyll and zooplankton at the mouth of the basin over the 25 h period. These variations were associated with changes in the height of the tide, but were about 2^1/2 h out of phase with it. Strong negative correlations were observed between chlorophyll and transport, such that only 35% of the chlorophyll exported was exchanged via the mean flow, while 65% was exchanged via the fluctuations. The correlation was even more striking with zooplankton, for which virtually all the export was associated with the fluctuations in the transport. Time series observations in the centre of the basin revealed considerable short-term variability in both chlorophyll and zooplankton, but the variations were smaller than those observed at the mouth of the basin, and the phase lag with the tide was longer. The variability studies enable suggestions to be made about more economical design of sampling programs, but illustrate the difficulty of providing verification data for any continuous model of primary production in such a basin.Bedford Institute Contribution No. 231.Canadian Contribution to IBP No. 97.     

Stimulation of phytoplankton production in coastal waters by natural rainfall inputs: Nutritional and trophic implications

Recent evaluations of estuarine and coastal nutrient budgets implicate atmospheric deposition as a potentially significant (20 to 30%) source of biologically available nitrogen. We examined the potential growth stimulating impact of atmospheric nitrogen loading (ANL), as local rainfall, in representative shallow, nitrogen limited North Carolina mesohaline estuarine and euhaline coastal Atlantic Ocean habitats. From July 1988 to December 1989, using in situ bioassays, we examined natural phytoplankton growth responses, as¹⁴CO₂ assimilation and chlorophylla production, to rain additions over a range of dilutions mimicking actual input levels. Rainfall at naturally occurring dilutions (0.5 to 5%) stimulated both¹⁴CO₂ assimilation and chlorophylla production, in most cases in a highly significant manner. Parallel nutrient enrichments consistently pointed to nitrogen as the growth stimulating nutrient source. Generally, more acidic rainfall led to greater magnitudes of growth stimulation, especially at lower dilutions. Nutrient analyses of local rainfall from May 1988 to January 1990 indicated an inverse relationship between pH and NO ₃ ^- content. There have been growing concerns regarding increasing coastal and estuarine eutrophication, including ecologically and economically devastating phytoplankton blooms bordering urban and industrial regions of North America, Europe, Japan, and Korea. It appears timely, if not essential, to consider atmospheric nutrient loading in the formulation and implementation of nutrient management strategies aimed at mitigating coastal eutrophication.     

Interspecific differences in the photosynthetic carbon metabolism of marine phytoplankton

Six species of marine phytoplankton of different sizes and taxonomic categories were grown in microcosms under identical experimental conditions; the species cultured were: Pavlova lutheri (Prymnesiophyceae), Dunaliella tertiolecta (Chlorophyceae), Phaeodactylum tricornutum (Baciollariophyceae), Eutreptiella sp. (Euglenophyceae), Alexandrium tamarense (Dinophyceae), and Phaeocystis pouchetii (Prymnesiophyceae). The photosynthetic carbon metabolism of these phytoplankton was studied throughout the exponential and lag phases of growth after nutrient depletion. The relative incorporation of carbon into protein was positively correlated with phytoplankton growth, while carbon assimilation into low molecular weight metabolites (LMWM) and storage products, i.e., lipid and polysaccharides, generally increased under nutrient-limiting conditions. Clear taxonspecific differences were observed in the proportions of carbon incorporated into cell constituents. A significant linear relationship was consistently found between the relative synthesis of protein to LMWM, and both the production normalised to chlorophyll (P:B) and the phytoplankton growth rate. However, ANCOVA revealed significant, interspecific differences in these relationships.     

The effects of seston food quality on planktonic food web patterns

In planktonic food webs, the conversion rate of plant material to herbivore biomass is determined by a variety of factors such as seston biochemical/elemental composition, phytoplankton cell morphology, and colony architecture. Despite the overwhelming heterogeneity characterizing the plant–animal interface, plankton population models usually misrepresent the food quality constraints imposed on zooplankton growth. In this study, we reformulate the zooplankton grazing term to include seston food quality effects on zooplankton assimilation efficiency and examine its ramifications on system stability. Using different phytoplankton parameterizations with regards to growth strategies, light requirements, sinking rates, and food quality, we examined the dynamics induced in planktonic systems under varying zooplankton mortality/fish predation, light conditions, nutrient availability, and detritus food quality levels. In general, our analysis suggests that high food quality tends to stabilize the planktonic systems, whereas unforced oscillations (limit cycles) emerge with lower seston food quality. For a given phytoplankton specification and resource availability, the amplitude of the plankton oscillations is primarily modulated from zooplankton mortality and secondarily from the nutritional quality of the alternative food source (i.e., detritus). When the phytoplankton community is parameterized as a cyanobacterium-like species, conditions of high nutrient availability combined with high zooplankton mortality led to phytoplankton biomass accumulation, whereas a diatom-like parameterization resulted in relatively low phytoplankton to zooplankton biomass ratios highlighting the notion that high phytoplankton food quality allows the zooplankton community to sustain relatively high biomass and to suppress phytoplankton biomass to low levels. During nutrient and light enrichment conditions, both phytoplankton and detritus food quality determine the extent of the limit cycle region, whereas high algal food quality increases system resilience by shifting the oscillatory region towards lower light attenuation levels. Detritus food quality seems to regulate the amplitude of the dynamic oscillations following enrichment, when algal food quality is low. These results highlight the profitability of the alternative food sources for the grazer as an important predictor for the dynamic behavior of primary producer–grazer interactions in nature.     

Mechanisms of production and fate of organic phosphorus in the northern Adriatic Sea

In the period from 1980 to 1984 organic phosphorus, nutrients, primary production rates (¹⁴C), chlorophyll a (chl a) standing crops, and basic oceanographic parameters were measured during 23 cruises at six stations in the open waters of the northern Adriatic Sea. These waters are significantly influenced by polluted Po River discharge. Organic phosphorus was correlated with several parameters which characterize phytoplankton activity and organic matter decomposition processes. In the late winter-spring period, organic phosphorus is produced during phytoplankton blooms. It is hypothesized that microzooplankton grazing is the main factor increasing the organic phosphorus concentrations in summer (up to 1.1 mol 1^-1). Fall and winter had much lower values (below 0.3 mol 1^-1) due to remineralization processes and an increased water mass exchange between the northern and central Adriatic regions. The direct contribution of organic phosphorus by freshwater discharge was not found to be significant. The higher organic phosphorus concentrations that can occur in low salinity waters are most likely due to their increased capability to support primary production.     

Production of DMSP and DMS during a mesocosm study of an Emiliania huxleyi bloom: influence of bacteria and Calanus finmarchicus grazing

We investigated the influence of bacteria and metazooplankton on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seawater mesocosms. The phytoplankton succession was marked by the rapid collapse of an initial Skeletonema costatum (Greville) Cleve bloom followed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSP_d) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was followed by a rapid decrease in DMSP_d concentrations which remained low for the rest of the experiment, even during the subsequent collapse of the E. huxleyi blooms. The absence of DMSP_d and DMS peaks during the declining phase of the E. huxleyi blooms was attributed to the high bacterial activity prevailing at that time. The influence of metazooplankton grazing on DMSP and DMS production was investigated by adding moderate (24 mg dry weight m^-3) and high (520 mg dry weight m^-3) concentrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 m mesh net) enclosures during the E. huxleyi blooms. The addition of C. finmarchicus, even in high concentrations, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulation of chlorophyll a and particulate DMSP. These results suggest that copepods were preying on the herbivorous microzooplankton which, in turn, was controlling the biomass of nanophytoplankton. DMS production was also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton containing DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude of DMS pulses following phytoplankton blooms.