MEPS 639:53-71 (2020)  -  DOI: https://doi.org/10.3354/meps13267

ABSTRACT: Pelagic primary production measurements provide fundamental information about the trophic status of a marine ecosystem. Measured carbon fixation rates generally have a limited temporal and spatial resolution, but can be combined with Earth Observation data to extrapolate the measurements. Here, P-E curves were fitted for 3 yr of ¹⁴C incubation data from the western Wadden Sea, using 4 different models; 2 with and 2 without photo-inhibition. The curve-fit model by Jassby & Platt (1976) best fit the data. Applying this model showed that the photosynthetic parameters, normalised for chlorophyll a concentration, of maximum production (P^B_max) and initial slope of the P-E curve (α^B) were correlated. Seasonality in photosynthetic parameters of this model and the relationship with environmental variables were explored, with a focus on variables that can be inferred from satellite algorithms. There were no significant correlations between α^B and any of the environmental variables measured. While P^B_max correlated with sea surface temperature (SST), the vertical light attenuation coefficient, silicate and nitrate + nitrite concentration, the multivariate model that best explained the variation in estimates of P^B_max was a model that included SST and year. In the period from 2012-2014, daily and annual production ranged between 3.4 and 3800 mg C d^-1 and between 131 and 239 g C m^-2 yr^-1, respectively. Comparison of these results with historical data (1990-2003) indicates that the decline in planktonic primary production that has been happening since the 1990s has halted. Although not tested, we believe that our approach is generally applicable to coastal waters.

KEY WORDS: Production-light curve · Photosynthetic parameters · Environmental variables · ¹⁴C incubations · Phytoplankton