Gross and net primary production in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988

Summary Daily rates of gross and net primary production were calculated in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988 (EPOS, Leg 2) on the basis of kinetic experiments, which combine radiotracer technology and classic biochemical procedures, and by taking into account the light regime, the physical structure of the water column, the vertical distribution of chlorophyll a, and the protozoan grazing pressure. From these calculations, three distinct sub-areas were identified: the Closed Pack Ice Zone (CPIZ), characterized by the lowest average gross primary production (0.36 gC · m^–2 · day^–1); the Marginal Ice Zone (MIZ) with a maximum mean value of 1.76 gC · m^–2 · day^–1; and the Open Ocean Zone off the ice edge (OOZ) with an intermediate mean value of 0.87 gC · m^–2 · day^–1. Net primary production fluctuated nearly in the same proportions, averaging 0.55, 0.2 and 1.13 gC · m^–2 · day^–1 in the OOZ, CPIZ and MIZ respectively, representing 53% of the total photo-assimilated carbon under heavy ice cover (CPIZ) and 64% in the two other areas. Available light, strongly dependent on the ice cover, was shown to control the level of primary production in the sea ice associated sub-areas, whilst protozoa grazing on phytoplankton determined the moderate primary production level characteristic of the well illuminated OOZ area.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Phytoplankton production and biomass at frontal zones in the Atlantic sector of the Southern Ocean

A high resolution study of chlorophyll a and primary production distribution was carried out in the Atlantic sector of the Southern Ocean during the austral summer of 1990–91. Primary production (¹⁴C assimilation) and photosynthetic capacity levels at frontal systems were among the highest recorded during the cruise (2.8–6.3 mgC·m^–3·h^–1, and 1.3–4.7mgC·mgChl a ^–1·h^–1, respectively). Blooms at ocean fronts were strongly dominated by specific size classes and species. This suggests that the increase in biomass was probably the result of an enhancement of in situ production by selected components of the phytoplankton assemblage, rather than accumulation of cells through hydrographic forces. This hypothesis is supported by the high variability of photosynthetic capacities at adjacent stations along the transects. Blooms (ca 2.7–3.5 mg Chl a·m^–3) were found at three oceanic fronts (the Subtropical, Subantarctic and Antarctic Polar Fronts) during the early summer. These were equivalent to, or denser than, blooms in the Marginal Ice Zone and at the Continental Water Boundary. Seasonal effects on phytoplankton community structure were very marked. In early summer (December), netphyto-plankton (>20 m) was consistently the major component of the frontal blooms, with the chain-forming diatoms Chaetoceros spp. and Nitzschia spp. dominating at the Subantarctic and Antarctic Polar Fronts, respectively. During late summer (February), nanophytoplankton (1–20 m) usually dominated algal communities at the main frontal areas. Only at the Antarctic Polar Front did netphytoplankton dominate, with the diatom component consisting almost exclusively of Corethron criophilum. An early to late summer shift of maximum phytoplankton biomass from north to south of the Antarctic Polar Front was observed. Spatial covariance between silicate levels and water-column stability appeared to be the main factor controlling phytoplankton production at the Antarctic Polar Front. Low silicate concentrations may have limited diatom growth at the northern edge of the front, while a deep mixed layer depth reduced production at the southern edge of the front.     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.     

Phosphorus and nitrogen excretion rates of zooplankton from the eutrophic Loosdrecht lakes,with notes on other P sources for phytoplankton requirements

Phosphorus and nitrogen excretion rates by zooplankton communities from two eutrophic and shallow Dutch lakes were measured in laboratory. The variations in excretion rates in the lakes (May–October) were caused mainly by fluctuation in zooplankton biomass. Mean summer excretion rates (June–September) were 2.4 and 0.9 µg PO₄P·1^–1·d^–1 in Lake Loosdercht and Lake Breukeleveen, respectively. This difference between the lakes was caused mainly by the lower zooplankton biomass in Lake Breukeleveen. The excretion of 2.4 µg PO₄P·1^–1·d^– compared with the calculated P-demand of phytoplankton of 8.0 µg PO₄P·1^–1·d^–1 is substantial in the summer (June–September) and far more important than the external P-supply of 0.4 µg P·1^–1·d^–1 and sediment release of 0.5 µg P·1^–1·d^–1. Both temperature and composition of zooplankton affected the weight specific excretion rates of the zooplankton community. The weight specific community excretion rates of P and N increased with temperature (exponential model); 1–8 g PO₄P·mg^–1 zooplankton-C·d^–1 and 5–42 µg NH₃N·mg^–1 zooplankton-C·d^–1 (10°C–20°C).     

Primary production and phytoplankton in two small,polyhumic forest lakes in southern Finland

Primary production and phytoplankton in polyhumic lakes showed a very distinct seasonal succession. A vigorous spring maximum produced by Chlamydomonas green algae at the beginning of the growing season and two summer maxima composed mainly of Mallomonas caudata Iwanoff were typical. The annual primary production was ca. 6 g org. C · m^–2 in both lakes. The mean epilimnetic biomass was 1.1 in the first lake and 2.2 g · m^–2 (ww) in the second one. The maximum phytoplankton biomass, 14 g · m^–2, was observed during the vernal peak in May.     

Excretion of photosynthetically fixed organic carbon by metalimnetic phytoplankton

The effects of light intensity, oxygen concentration, and pH on the rates of photosynthesis and net excretion by metalimnetic phytoplankton populations of Little Crooked Lake, Indiana, were studied. Photosynthetic rates increased from 1.42 to 3.14 mg C·mg^–1 chlorophylla·hour^–1 within a range of light intensities from 65 to 150E·m^–2·sec^–1, whereas net excretion remained constant at 0.05 mg C·mg^–1 chlorophylla·hour^–1. Bacteria assimilated approximately 50% of the carbon released by the phytoplankton under these conditions. Excreted carbon (organic compounds either assimilated by bacteria or dissolved in the lake water) was produced by phytoplankton at rates of 0.02–0.15 mg C·mg^–1 chlorophylla·hour^–1. These rates were 6%–13% of the photosynthetic rates of the phytoplankton. Both total excretion of carbon and bacterial assimilation of excreted carbon increased at high light intensities whereas net excretion remained fairly constant. Elevated oxygen concentrations in samples incubated at 150E· m^–2·sec^–1 decreased rates of both photosynthesis and net excretion. The photosynthetic rate increased from 3.0 to 5.0 mg C·mg^–1 chlorophylla· hour^–1 as the pH was raised from 7.5 to 8.8. Net excretion within this range decreased slightly. Calculation of total primary production using a numerical model showed that whereas 225.8 g C·m^–2 was photosynthetically fixed between 12 May and 24 August 1982, a maximum of about 9.3 g C·m^–2 was released extracellularly.     

Submerged Macrophyte-bed Effects on Water-Column Phosphorus,Chlorophyll <Emphasis Type="Italic">a</Emphasis>, and Bacterial Production

Submerged macrophytes are a major component of freshwater ecosystems, yet their net effect on water column phosphorus (P), algae, and bacterioplankton is not well understood. A 4-month mass-balance study during the summer quantified the net effect of a large (5.5 ha) undisturbed macrophyte bed on these water-column properties. The bed is located in a slow-flowing (0.05–0.1 cm s^–1) channel between two lakes, allowing for the quantification of inputs and outputs. The P budget for the study period showed that, despite considerable short-term variation, the macrophyte bed was a negligible net sink for P (0.06 mg m^–2 day^–1, range from –0.76 to +0.79 mg m^–2 day^–1), demonstrating that loading and uptake processes in the weedbed roughly balance over the summer. Chlorophyll a was disproportionately retained relative to particulate organic carbon (POC), indicating that the algal component of the POC was preferentially trapped. However, the principal contribution of the weedbed to the open water was a consistent positive influence on bacterioplankton production over the summer. Conservative extrapolations based on measured August specific exports (m^–2 day^–1) of P and bacterial production exiting the weedbed applied to five regional lakes varying in lake morphometry and macrophyte cover suggest that even in the most macrophyte dominated of lakes (66% cover), P loading from submerged weedbeds never exceeds 1% day^–1 of standing epilimnetic P levels, whereas subsidization of bacterioplankton production can reach upward of 20% day^–1. The presence of submerged macrophytes therefore differentially modifies algae and bacteria in the water column, while modestly altering P dynamics over the summer.     

The annual flood regime as a regulatory mechanism for phytoplankton production in Kainji lake,Nigeria

Phytoplankton production was measured in situ in Kainji lake from December 1970 to September 1972 using the oxygen light and dark bottle technique. Seasonal variations in solar radiation, transparency, temperature, and composition of subsurface light were also measured. Oxygen production per unit area varied from 220 to 4500 mg O₂ m^–2 day^–1, the maximum production rate from 95 to 400 mg O₂ m^–3 h^–1. Seasonal mixing of lake water and river water of varying turbidity changed the optical properties of the lake water and consequently affected phytoplankton production. The annual flood pattern was found to be an important factor regulating phytoplankton production in the lake.     

Phytoplankton biomass and primary production in semi-enclosed reef lagoons of the central Great Barrier Reef,Australia

Phytoplankton biomass and primary production rates within semi-enclosed reef lagoons of the central Great Barrier Reef were compared with adjacent shelf waters. Chlorophyll concentrations and surface primary production rates were usually higher in lagoons although seasonal differences were only significant during the summer. Nitrate concentrations were higher in lagoons than in shelf waters year-round. Nano- (<20 m size fraction) or pico-phytoplankton (<2 m size fraction) dominated phytoplankton biomass and production within reef lagoons throughout the year. Net phytoplankton (>10–20 m size fraction), however, were relatively more important in both reef lagoons and open shelf waters during the summer. Biomass-specific production within lagoons (range 41–90 mg C mg chl^–1 day^–1) was high, regardless of season. Lagoonal phytoplankton production (range 0.2–1.6 g C m^–2 day^–1) was directly correlated with standing crop and inversely related to lagoon flushing rates. Phytoplankton blooms develop within GBR reef lagoons during intermittent calm periods when water residence times exceed phytoplankton generation times.     

Light-limited algal growth in Lake Loosdrecht: steady state studies in laboratory scale enclosures

Phytoplankton growth in the shallow, turbid Lake Loosdrecht (The Netherlands) is importantly influenced by light availability, and thus the concentrations of the various light-attenuating materials. The system is highly eutrophic and supports an algal biomass of ca. 160 mg Chl m^–3. A model is proposed here which predicts algal growth in the lake as a function of the light received and subsequent attenuation in the water column by phytoplankton, tripton and background colour. The model is based on an energy balance which relates growth rate to the true growth yield on light energy and the energy demand for cell maintenance. The coefficients for energy conversion (Y = 0.002 gDW kJ^–1) and cell maintenance (µ_e = 0.031 day^–1) were determined from steady state growth kinetics of Prochlorothrix hollandica in light-limited laboratory flow systems with the same depth as the lake and receiving summer average conditions of irradiance. Light attenuation by phytoplankton and tripton were quantified using specific attenuation coefficients: 0.011 m² mg^–1 Chl for the phytoplankton and 0.23 m² g^–1 DW for tripton.The growth studies demonstrated that Lake Loosdrecht can support a much higher algal biomass in the absence of non-algal particulate matter. The proposed model is used to predict chlorophyll a concentrations in dependence on growth rate and levels of tripton. Since approximately 75% of the sestonic dry weight in Lake Loosdrecht may be attributed to tripton, it is concluded that the algal biomass is markedly lowered by the abundance of tripton in the water column. A knowledge of the sources and fate of tripton in the lake is thus of fundamental importance in modelling phytoplankton dynamics.