Pj le B Williams

Review of gross community production primary production net community production and dark community respiration in the Gulf of Lions

Abstract Thirty years of data were used to examine the influence of the Rhoˆne River discharge on the spatio-temporal distribution of primary production in the Gulf of Lions. These patterns are characterised for four hydrological provinces: (i) the Gulf of Marseilles, a coastal oligotrophic system exhibiting a typical diatom-based bloom succession; (ii) the mouth and plume of the Rhoˆne River, containing high levels of nutrients, constant levels of chlorophyll a , and a plankton assemblage dominated by nanoflagellates; (iii) the highly productive dilution area to the west of the Rhoˆne, with a diatom spring bloom and smaller winter diatom blooms; (iv) the southern area encompassing the oligotrophic Liguro-Provencal Current, a frontal zone of constantly high primary production and a divergence area exhibiting a typical spring bloom situation. No evidence of eutrophication due to river input was seen. In situ and in vitro net community production data suggest the Gulf of Lions as a whole is a balanced ecosystem on an annual time scale, with the mean primary production lying between 78 and 142 g C m −2 a −1 .

Water column and sea-ice primary production during Austral spring in the Bellingshausen Sea

Abstract The findings of a cruise to study the phytoplankton bloom dynamics associated with the marginal ice zone (MIZ) in the Bellingshausen Sea during Austral spring (November-December) 1992 are reported. Biomass and rate process measurements were carried out at stations located in the ice, ice edge and open water along the 85°W meridian in order to establish the productivity of the microalgae associated with sea-ice and in the water column. In addition, a series of transects along 85°W from sea-ice to open water conditions enabled an assessment of the development of phytoplankton populations. Low phytoplankton biomass and production were noted at ice-covered and ice-edge stations and in the open water close to the ice edge. Observations from the transects indicated no development of a classical ice edge bloom despite evidence that sea-ice had retreated more than 100 km during the study period. Survey data along the 85°W line revealed a region of high chlorophyll, centred on 67.5°S, which was initially observed during brash ice conditions. This feature, which remained geographically consistent, persisted for at least 25 days and was thought to be associated with a frontal region. Water column primary production ( 14 C) in this high chlorophyll region was ca 0.8 g C m −2 day − , more than 8 times higher than noted in the MIZ. Phytoplankton photosynthetic characteristics within this region indicated that cells were adapted to a low light regime. A critical depth of 80 m, estimated directly from oxygen flux measurements, was sufficient to permit the initiation and net growth of phytoplankton standing stocks in a mixed layer of ca 70 m. A modelling approach using 14 C observations suggested that phytoplankton growth was less than the sum of the algal loss terms within this feature. An advective supply of cells therefore would be required to sustain the observed high and constant algal biomass. In addition, although this high chlorophyll feature was initially observed during brash-ice conditions, the available data suggest that it was initiated under open water conditions.

The PRIME 1996 cruise: an overview

Abstract The UK PRIME cruise, June–July 1996 in the NE Atlantic, consisted of two legs. During the first, detailed chemical and biological observations were made in time-series mode adjacent to the centre of a cold-core eddy in the vicinity of 59°N 20°W using SF6 tracer techniques as the basis for the Lagrangian study. The eddy, which appeared to have been formed the previous winter, remained coherent over the 9 days of the survey and advected only slowly. The phytoplankton community in the eddy was dominated by the coccolithophorid Coccolithus huxleyi. High microzooplankton grazing rates indicated minimal export losses from the surface layer. Significant shifts in many, but not all, of the chemical and biological properties measured were observed over the course of the experiment, especially after the passage of a storm event, which resulted in considerable deepening of the mixed layer followed by a return to fully stratified conditions. The second leg consisted of a transect from 59°N 20°W to 37°N 19°W, with a further Lagrangian time-series study based on a drogue marker initiated at the southern end of the transect. Maximal biological activity was generally encountered in the region between two fronts located at 52.5°N and 48°N, while to the south of 48°N oligotrophic conditions prevailed. At the southern Lagrangian site, a deep chlorophyll maximum was present and high column new production was recorded as a result of the euphotic zone extending below the depth of the nutricline. Microzooplankton grazing rates were lower at this location than at the northern eddy site. The influx of a warm, saline water body into the upper layers during the southern survey led to a major shift in many of the biological and chemical properties being measured. At both the northern and southern Lagrangian sites, the biomass of the mesozooplankton exceeded that of the microzooplankton.

The temperature response of gross and net community production and respiration in time-varying assemblages of temperate marine micro-plankton

Abstract Micro-organism community respiration and net community production rates and their response to temperature were determined as oxygen flux rates in the Menai Strait during a 6-month period including the spring and summer of 1993. The rates for gross community production were calculated from polynomial fits of community respiration and net community production data. The cardinal temperatures of gross community production were estimated from these equations. The optimal temperature was positively correlated to the in situ temperature. The natural population gave no evidence of being shocked due to experimental temperature manipulation. Frequency histograms of the temperature coefficients of community respiration and gross community production were distinct in this environment. Q 10 values for respiration were greater than Q 10 values for photosynthesis, in contrast to published observations from the Southern Ocean where they overlapped. It was argued that this was a consequence of the short-term temperature variability of the environment.

Physical and anthropogenic effects on observed long-term nutrient changes in the Irish Sea

Abstract The trend in Irish Sea nutrient concentrations over the last four decades has been considered to reflect changes in anthropogenic loading. Comparison of a long-term database for the Menai Strait, North Wales, with an established historic data set for the Cypris station, Isle of Man, indicates that climate also has a significant influence on observations of nutrient concentrations. Data are presented detailing long-term shifts in nitrate, phosphate and silicate measurements since the 1960s at these two fixed sampling sites in the Irish Sea. Broad systematic changes observed in all three nutrients over the decades show a rise from the 1960s through to the 1980s, followed generally by an overall decline in the 1990s. Decadal-scale salinity changes occur in the opposite sense to nutrient changes. Anthropogenic inputs from freshwater cannot fully account for observed nutrient trends, neither is there evidence for shifts in nutrient concentrations in oceanic waters over the past four decades. Climatically forced movement in the geographical position of the freshwater/seawater mixing zone over a decadal time scale could, however, give rise to the observed shifts in nutrient concentration and salinity. This cannot alter nutrient concentration and salinity per se, but causes the measurements taken at fixed sampling sites to fluctuate inversely over this time scale. It is concluded that there is complex interplay between anthropogenic loading and climate affecting the distribution of nutrients in the Irish Sea.

10.15 – Plankton Respiration, Net Community Production and the Organic Carbon Cycle in the Oceanic Water Column

Planktonic respiration is considered in the context of the overall oceanic organic carbon cycle. It is generally concluded that the overall oceans are a net sink for organic material, as estimates of organic input into the ocean from the rivers exceeds the net loss to the sediments. Thus, to maintain organic balance, biological removal (respiration) must exceed production (photosynthesis), i.e. the oceans as a whole must be net heterotrophic. Contemporary analyses conclude that the coastal zones are net autotrophic, exporting organic material to the open oceans, which as a consequence must be net heterotrophic. Resolving the open ocean budget within the major vertical zones (the epipelagic, mesopelagic and bathypelagic zones) has previously been achieved by estimating the net fluxes between them. The review supplements this approach by analysing the internal fluxes, derived from field measurements of respiration and net community production. If the whole data set in adopted uncritically the ranges of estimates is disturbingly wide. If, however, improbable measurements are disregarded then a largely consistent set of rates results. Theoretical and experimental approaches are used to determine the distribution of respiration within the planktonic community and show that 90–95% of respiration lies with organisms less than 100 μm.