Planktonic primary production in the German Wadden sea

Abstract

By combining weekly data of irradiance, attenuation and chlorophyll a concentrations with photosynthesis (P) versus light intensity (E) curve characteristics, the annual cycle of planktonic primary production in the estuarine part of the Northfrisian Wadden Sea was computed for a 2 year period. Daily water column particulate gross production ranged from 5 to 2200 mg C m-2 day-1 and showed a seasonal pattern similar to chlorophyll a. Budget calculation yielded annual gross particulate primary productions of 124 and 176 g C m-2 year-1 in 1995 and 1996, respectively. Annual amounts of phytoplankton respiration, calculated according to a two-compartment model of Langdon [in Li,W.K.W. and Maestrini, S.Y. (eds), Measurement of Primary Production from the Molecular to the Global Scale. International Council for the Exploration of the Sea, Copenhagen, 1993, pp. 20-36], and dissolved production in 1996, were both in the range of 24-39 g C m-2 year-1. Annual total net production was thus very similar to particulate gross production (127 and 177 g C m-2 year-1 in 1995 and 1996, respectively). Phytoplankton growth was low or even negative in winter. In spring and summer, production/biomass (Pr/B) ratios varied from 0.2 up to 1.7. Phytoplankton growth during the growth season always surpassed average flushing time in the area, thus underlining the potential of local phytoplankton bloom development in this part of the Wadden Sea. The chlorophyll-specific maximum photosynthetic rate (P(B)(max)) ranged from 0.8 to 9.9 mg C mg-1 Chl h-1 and was strongly correlated with water temperature (r2 = 0.67). By contrast, there was no clear seasonal cycle in α(B), which ranged from 0.007 to 0.039 mg C mg-1 Chl h-1 (μmol photons m-2 s-1)-1. Its variability was much less than P(B)(max) and independent of temperature. The magnitude and part of the variability of P(B)(max) and α(B) are presumably caused by changes in species composition, as evidenced from the range of these parameters found among 10 predominant diatom species isolated from the Wadden Sea. The ratio of average light conditions in the water column (E(av)) to the light saturation parameter E(k) indicates that primary production in the Wadden Sea region under study is predominantly controlled by light limitation and that nutrient limitation was likely to occur for a few hours per day only during 5 (dissolved inorganic nitrogen) to 10 (PO4, Si) weeks in the 2 year period investigated.