Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

Contribution of salps to carbon flux of marginal ice zone of the Lazarev Sea, southern ocean

In order to estimate the in situ grazing rates of Salpa thompsoni and their implications for the development of phytoplankton blooms and for the sequestration of biogenic carbon in the high Antarctic, a repeat-grid survey and drogue study were carried out in the Lazarev Sea during austral summer of 1994/1995 (December/January). Exceptionally high grazing rates were measured for S. thompsoni at the onset of a phytoplankton bloom (0.2 to 0.8 μg chlorophyll a l⁻¹) in December 1994, with up to ≃160 μg of plant pigments consumed by an individual salp of 7 to 10 cm length per day. Dense salp swarms extended throughout the marginal ice zone, consuming up to 108% of daily phytoplankton production and 21% of the total chlorophyll a stock. Due to the much faster sinking rates and higher carbon content of salp faecal pellets, the efficiency of downward carbon flux through salps is much higher than through the other major grazers, krill and copepods. S. thompsoni can thus export large amounts of biogenic carbon from the euphotic zone to the deep ocean. With the observed ingestion rates during December 1994, this flux could have attained levels of up to 88 mg C m⁻² d⁻¹, accounting for the bulk of the vertical transport of carbon in the Lazarev Sea. However, in January 1995, when phytoplankton concentrations exceeded a threshold level of 1.0 to 1.5 μg chlorophyll a l⁻¹, salps experienced a drastic reduction in their feeding efficiency, possibly as a result of clogging of their filtering apparatus. This triggered a dramatic reversal in the relationship, during which a dense phytoplankton bloom developed in conjunction with the collapse of the salp population. Increases in the biomass and geographic range of the tunicate S. thompsoni have occurred in several areas of the southern ocean, often in parallel with a rise in sea-surface temperature during sub-decadal periods of warming anomalies. Received: 10 August 1997 / Accepted: 21 October 1997     

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.     

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.     

Patterns of microzooplankton growth in dilution experiments across a trophic gradient: Implications for herbivory studies

To investigate the growth and grazing patterns of microzooplankton (MZP) in environments of differing productivity, dilution experiments measuring phytoplankton growth (μ) and grazing mortality (m) rates were performed using samples from contrasting locations along the Texas coast. Samples were collected from estuaries, coastal lagoons and offshore Gulf of Mexico locations in the spring and summer of 2001. MZP growth rates were determined in each dilution treatment. Although MZP biomass changed over time in most dilution treatments, adjusting μ and m for the actual grazer gradient (represented by geometric mean MZP biomass) did not cause a significant deviation from the nominal dilution gradient. Likewise, these adjustments did not yield significant regressions where none existed before adjustment. The dynamics of MZP taxonomic groups (ciliates, dinoflagellates) and size categories differed suggesting that in some cases internal predation may lead to trophic cascades. MZP biomass was higher in productive coastal waters and included a larger proportion of dinoflagellates than in the oligotrophic, ciliate-dominated waters of the Gulf of Mexico. The MZP biomass-to-chlorophyll a ratio was lowest in the hypereutrophic Nueces River, where MZP biomass significantly increased in all dilution treatments (net growth rates up to 2 day⁻¹) suggesting a strong top–down control. In the brown-tide dominated Upper Laguna Madre and the oligotrophic seagrass-dominated Lower Laguna Madre MZP growth was decoupled from that of phytoplankton. At these sites, MZP were likely fueled by bacterial carbon and mixotrophy, respectively. Observing the growth response of MZP in dilution experiments can provide insight into trophic structure and efficiency of the microbial food web.     

Nitrate storage by phytoplankton in a coastal upwelling environment

The storage of nitrate by phytoplankton cells during the early phases of upwelling was studied in coastal stations off northern Spain (southern Bay of Biscay) between 1990 and 1994. In this region, a persistent upwelling during summer is characterised by intermittent pulses of variable intensity, and increased nutrient concentrations in the surface layer. The main effect of an upwelling pulse on phytoplankton distribution is the shifting of the chlorophyll a and primary production maxima to near the surface. When the upwelling relaxes, thermal stratification of the water column occurs, and a distinct subsurface chlorophyll maximum develops below the production maximum. An accumulation of intracellular nitrate characterized the early phases of upwelling (mean = 2.73 μmol N m⁻³), maximum concentrations being attained at depths where biomass and production values were moderate. In contrast, phytoplankton cells from non-upwelling situations contained significantly lower concentrations of intracellular nitrate (mean = 0.17 μmol N m⁻³). The variations in the intracellular pool of nitrate may result from the differential allocation of resources within the cell as a result of variations in the energy available, since the uptake and assimilation of nitrate is a relatively expensive process involving several enzymatic systems. We hypothesize that nitrate storage by phytoplankton cells is characteristic of early phases of upwelling and is linked to patterns of carbon fixation. Average nitrogen budgets for upwelling and non-upwelling situations indicate that intracellular nitrate reserves are not responsible for maintaining high phytoplankton growth rates, since they only account for <2% of daily primary production during upwelling events. Received: 28 August 1996 / Accepted 3 December 1996     

Grazing of phytoplankton by copepods in eastern Antarctic coastal waters

Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke‐out. Phytoplankton biomass increased again from mid‐ to late‐February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break‐out, whereas larval polychaetes and ctenophores dominated after ice break‐out. Oncaeacurvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C␣m⁻³␣d⁻¹) was recorded on 21 December when O.␣curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m⁻³ d⁻¹); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February (⋍5 mg C␣m⁻³␣d⁻¹). The main species responsible for grazing during this period were O. curvata, Oithonasimilis, Calanoidesacutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity. Received: 11 October 1996 / Accepted: 22 October 1996     

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.     

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.     

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.     

Production and biomass accumulation of periphytic diatoms growing on glass slides during a 1-year cycle in a subtropical estuarine environment (Bay of Paranaguá, southern Brazil)

Production rates, chlorophyll concentrations and general composition of periphytic diatom communities growing on glass slides were studied in relation to environmental parameters during one seasonal cycle in the Bay of Paranaguá, southern Brazil. Slides were routinely submersed at 1, 2 and 3 m depth and recovered weekly for microscopic examinations, analyses of chlorophyll, cell counts and in situ photosynthetic incubations using the Winkler titration method. Water samples were also collected at surface and bottom layers for determinations of temperature, salinity, nutrients and chlorophyll in the water. The periphytic community was mainly formed by epipelic and epipsammic species, dominated by Navicula phyllepta, Cylindrotheca closterium, Navicula spp. and Amphora sp. Weekly chlorophyll a and cell accumulations on slides varied from <1–32 mg m⁻² and up to 31 × 10⁸cells m⁻², respectively. Photosynthetic rates varied from <1 to 35 mg oxygen mg chlorophyll a ⁻¹ h⁻¹, with higher values in summer. Daily production varied from 5 to 3,600 mg oxygen m⁻² day⁻¹ (<0.01–1.4 g carbon m⁻² day⁻¹). Multiple regression analysis revealed that vertical differences in light conditions and grazing pressure jointly affected the influence of temperature on the seasonal patterns of cell densities and chlorophyll concentrations according to depth. Received: 27 April 2000 / Accepted: 16 August 2000     

Influence of food quantity and temperature on development and growth of the marine copepod Calanus chilensis from northern Chile

An experiment under laboratory conditions was conducted to test the hypothesis that development and growth of copepodite stages in Calanus chilensis are temperature-dependent and not subject to food shortage in the upwelling area of the Humboldt Current, northern Chile. Field data obtained from June 1994 to May 1995 in Bahía Mejillones (23°S) were used to define four combinations of temperature and food under which copepodites were reared from Stage CIII to adulthood. The high temperature was 18.1 °C and the low temperature 13.1 °C, whereas the high food level was in the range of 6.8 to 24.8 μg l⁻¹ chlorophyll a and the low level 1.0 to 6.8 μg l⁻¹ chlorophyll a. As food a mixture of three unknown species of phytoflagellates and the diatom Navicula cryptocephala was used. This phytoplankton was initially obtained from the same sampling sites as copepods and kept in f/2 media at stable levels and composition throughout the experiment. The development rate (1/t), estimated from the time (t) elapsing between Stage CIV and adult, was significantly affected by both temperature and food, although low-food effects were much more remarkable. Low-food conditions also significantly reduced body length and “structural” (lipid-discounted) body mass at adulthood, while temperature only affected body length. The weight-specific growth rate was also affected by food and temperature, but again food effects were much more drastic. The results indicate that C. chilensis is a highly sensitive species to lack of food, and is possibly subject to food shortage during its annual cycle in the coastal upwelling area of northern Chile. Food limitation may help explain the seasonal pattern of adult size reported by previous studies in the area and the lack of consistence between the number of generations predictable from a temperature-dependent model and that observed in the field during the annual cycle. Received: 10 September 1996 / Accepted: 29 October 1996     

Interactions between NH+4 and NO−3 uptake and assimilation: comparison of diatoms and dinoflagellates at several growth temperatures

Ammonium concentrations of ∼1 M are commonly cited as being the threshold for inhibition of NO₃ ⁻ uptake, but the applicability of this threshold to phytoplankton from different taxonomic classes has rarely been examined. Additionally, little is known about the influence of environmental variables (e.g. growth temperature) on the interaction between ambient NH₄ ⁺ and NO₃ ⁻ uptake. Four species of estuarine phytoplankton, two diatom [Chaetoceros sp., and Thalassiosira weissflogii (Grunow) Fryxell et Hasle] and two dinoflagellate [Prorocentrum minimum (Pavillard) Schiller, and Gyrodinium uncatenum Hulburt], were grown on NO₃ ⁻ at several different temperatures (4, 10, 15, or 20 °C), and the impact of NH₄ ⁺ additions on NO₃ ⁻ uptake/assimilation (non-TCA-extracted) and assimilation (TCA-extracted) was assessed. For all species at all temperatures, NO₃ ⁻ uptake/assimilation and assimilation rates decreased in a roughly exponential manner with increasing NH₄ ⁺ concentrations but were not completely inhibited even at elevated NH₄ ⁺ concentrations of 200 μM. Estimated half-inhibition concentrations (K _i) were significantly greater in the diatom species (mean ± SE; 2.70 ± 0.67 μM) than in the dinoflagellate species (1.26 ± 0.55 μM). Half-inhibition constants were positively related to temperature-limited relative growth rate although not significantly. The observed inhibition of NO₃ ⁻ uptake and assimilation, as a percentage of NO₃ ⁻ uptake in the absence of NH₄ ⁺, averaged about 80% and ranged from 49 to 100%. For all species, a significant (P < 0.001) positive correlation was found between percent inhibition of NO₃ ⁻ assimilation and temperature-limited relative growth rate. Two experiments on Chesapeake Bay phytoplankton during an April 1998 diatom bloom showed that in short-term (∼1 h) temperature manipulation experiments, percent inhibition of NO₃ ⁻ uptake/assimilation was also positively related (P = 0.05) to experimental temperature. The observed relationships between temperature-limited relative growth rate and percent inhibition of NO₃ ⁻ assimilation rates for the species tested suggest that at the enzyme level, the inhibitory mechanism of NO₃ ⁻ assimilation is similar among species, but at the whole cell level may be regulated by species-specific differences in the accumulation of internal metabolites. These findings add not only to our understanding of species-specific variability and the role of growth temperature, but also provide additional data with which to evaluate current models of NH₄ ⁺ and NO₃ ⁻ interactions. Received: 31 August 1998 / Accepted: 7 December 1998     

Growth and grazing responses of tintinnid ciliates feeding on the toxic dinoflagellate Heterocapsacircularisquama

Growth and feeding rates of two tintinnid species, Favellaazorica and Favellataraikaensis, were determined under various concentrations of the dinoflagellate Heterocapsacircularisquama which has been reported as highly toxic to shellfish. Mean growth rates of F. azorica and F. taraikaensis on a diet of H.circularisquama (ca. 10² cells ml⁻¹) were 2.15 and 1.97 doublings d⁻¹, respectively. These values are similar to those on a diet of Heterocapsatriquetra which is suitable food for various zooplankton. However, growth rates of both tintinnid species decrease with increasing concentrations of >10³ cells ml⁻¹ of H. circularisquama. In particular, H. circularisquama under conditions of >10³ cells ml⁻¹ caused mortality in F.taraikaensis, probably due to toxins. Clearance and ingestion rates of F. azorica on H. circularisquama were 4.1 to 27.5 μl ind⁻¹ h⁻¹ and 1.5 to 28.7 cells ind⁻¹ h⁻¹, respectively, at concentrations of <10⁴ cells ml⁻¹ and those of F. taraikaensis were 0.9 to 22.1 μl ind⁻¹ h⁻¹ and 0.1 to 13.0 cells ind⁻¹ h⁻¹, respectively, at concentrations of <10³ cells ml⁻¹. Both clearance and ingestion rates on H.circularisquama were higher for replicates fed on H.triquetra. Daily grazing impact of the two species of Favella on the initial stage of a bloom of H.circularisquama were estimated to reach 6 to 50% of H. circularisquama at a concentration of 540 cells ml⁻¹, indicating that grazing by tintinnids such as Favella spp. may significantly regulate the initial stages of blooms of H. circularisquama. Received: 3 January 1997 / Accepted: 27 January 1997     

Standing crops and vertical distribution of four groups of marine planktonic ciliates in relation to phytoplankton chlorophyll a

Standing crops and the vertical distribution of four groups of ciliates, autotrophic naked ciliates (ANC), heterotrophic naked ciliates (HNC ), mixotrophic naked ciliates (MNC) and loricated ciliates (LC ), were analysed in relation to phytoplankton chlorophyll a in the western Pacific. Data were gathered from the upper 60 to 200 m of water in the subarctic North Pacific in spring, in the subtropical North Pacific in spring, in Toyama Bay in summer, off eastern Australia in spring and off Sanriku in spring and fall. Of the four groups, the standing crop of HNC (cells l⁻¹) showed the highest correlation to phytoplankton chloro- phyll a [CHL, μg l⁻¹; r=0.66, n=365 (HNC=490 ×CHL ^0.653)]. Depths of the maximum layers of HNC and MNC were usually shallower than that of chlorophyll a, while those of ANC and LC occurred frequently beneath the chlorophyll maximum layer. This indicates that these four ciliate groups are substantially different from each other, and that each group holds a different ecological position in the marine microbial food web. Received: 27 October 1997 / Accepted: 27 June 1998     

Annual cycles of phytoplankton community-structure and bloom dynamics in the Neuse River Estuary, North Carolina

The spatiotemporal distributions of major phytoplankton taxa were quantified to estimate the relative contribution of different microalgal groups to biomass and bloom dynamics in the eutrophic Neuse River Estuary, North Carolina, USA. Biweekly water samples and ambient physical and chemical data were examined at sites along a salinity gradient from January 1994 through December 1996. Chemosystematic photopigments (chlorophylls and carotenoids) were identified and quantified using high-performance liquid chromatography (HPLC). A recently-developed factor-analysis procedure (CHEMTAX) was used to partition the algal group-specific chlorophyll a (chl a) concentrations based on photopigment concentrations. Results were spatially and temporally integrated to determine the ecosystem-level dynamics of phytoplankton community-constituents. Seasonal patterns of phytoplankton community-composition changes were observed over the 3 yr. Dinoflagellates reached maximum abundance in the late winter to early spring (January to March), followed by a spring diatom bloom (May to July). Cyanobacteria were more prevalent during summer months and made a large contribution to phytoplankton biomass, possibly in response to nutrient-enriched freshwater discharge. Cryptomonad blooms were not associated with a particular season, and varied from year to year. Chlorophyte abundance was low, but occasional blooms occurred during spring and summer. Over the 3 yr period, the total contribution of each algal group, in terms of chl a, was evenly balanced, with each contributing nearly 20% of the total chl a. Cryptomonad, chlorophyte, and cyanobacterial dynamics did not exhibit regular seasonal bloom patterns. High dissolved inorganic-nitrogen loading during the summer months promoted major blooms of cryptomonads, chlorophytes, and cyanobacteria. Received: 12 September 1997 / Accepted: 12 December 1997     

The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). II. Abundance, biomass and production

 The abundance and biomass of Corophium multisetosum Stock, 1952 were determined from benthic corer samples collected monthly over 1 yr in the upper reaches of Canal de Mira (Ria de Aveiro, Portugal). Both density and biomass over the sampling period were negatively correlated with water temperature and positively correlated with chlorophyll a concentration in the sediment. C. multisetosum density was significantly negatively correlated with plant biomass and positively correlated with salinity. The nature of the sediment, favourable environmental conditions, high availability of food and low interspecific competition allowed the population to reach a maximal density of 200 × 10³ individuals m⁻² and a maximal biomass (ash-free dry wt, AFDW) of 62 g_AFDW m⁻². The population was highly productive, especially during the autumn/winter period. Production, estimated by two different methods (Hynes method: 251 g_AFDW m⁻² yr⁻¹; Morin–Bourassa method: 308 g_DW m⁻² yr⁻¹), was much higher than the values reported for other Corophium species. The annual P:Bˉ ratio (10) was high, but similar to values reported for Swedish populations of C. volutator and lower than the values estimated from Mediterranean populations of C. insidiosum. Received: 8 October 1999 / Accepted: 22 June 2000     

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     

Distribution and maximum growth depth of Fucus vesiculosus along the Gulf of Finland

 A survey of the distribution and maximum depth of a continuous Fucus vesiculosus belt was carried out in the Gulf of Finland in 1991. F. vesiculosus is widely distributed throughout the Gulf of Finland, including the vicinity of Vyborg Bay, Russia in the east. The maximum growth depth of F. vesiculosus in the Gulf of Finland reflects two different patterns according to the exposure to wave action. The most robust and continuous F. vesiculosus belt is observed on exposed shores, where the maximum growth depth is 5 to 6 m, with the optimum at 2 to 3 m. On moderately exposed shores the maximum growth depth is 3 m, with an optimum growth depth of <2 m. The maximum growth depth also varies geographically, with a decreasing trend towards the east. Maximum growth depth of F. vesiculosus correlates with light intensity. The compensation point for F. vesiculosus photosynthesis is about 25 μmol m⁻² s⁻¹, and photosynthesis is saturated at a light intensity of 300 μmol m⁻² s⁻¹. Vertical irradiance attenuation measurements in situ in summer revealed that for F. vesiculosus photosynthesis the quantity of light is optimal (200 to 300 μmol m⁻² s⁻¹) at <3 m depth. At depths >5 m the quantity of light is near or below the photosynthesis compensation point and insufficient for growth. These depth limits of light penetration coincide with measured growth depths of F. vesiculosus in the Gulf of Finland. Received: 7 May 1999 / Accepted: 18 November 1999     

Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima

Nutrients were added separately and combined to an initial concentration of 10 μM (ammonium) and/or 2 μM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations remained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and 12 mo periods for the first and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the clams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium (N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 · 10⁸ cell clam⁻¹, N = 6.6 · 10⁸ cell clam⁻¹, P = 5.7 · 10⁸ cell clam⁻¹, N + P = 5.7 · 10⁸ cell clam⁻¹; and C = 4.1 · 10⁸ cell clam⁻¹, N = 5.1 · 10⁸ cell clam⁻¹, P = 4.7 · 10⁸ cell clam⁻¹, N + P = 4.5 · 10⁸ cell clam⁻¹, at the end of the first and second phases of the experiment, respectively), although no differences in the mitotic index of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C = 7.0 mg chl a clam⁻¹, N = 13.1 mg chl a clam⁻¹, P = 12.9 mg chl a clam⁻¹, N + P = 11.8 mg chl a clam⁻¹; and C = 8.8 mg chl a clam⁻¹, N = 12.8 mg chl a clam⁻¹; P = 11.2 mg chl a clam⁻¹, N + P = 11.3 mg chl a clam⁻¹, at the end of the first and second phases of the experiment, respectively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam–zooxanthellae association. Received: 2 June 1997 / Accepted: 9 June 1997