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APPENDIX A. Estimation method primary production of phytoplankton in the WaddenSea
Primary production was measured in the laboratory in an incubator, kept at in situ temperature and constant light conditions, using the 14C method (Cadée and Hegeman 1974). In contrast to the original method (e.g. Cadée and Hegeman 1974, 2002) to estimate in situ production, Pin situ (gC m-2 d-1), from incubated production, Pincub (gC m-3 h-1), we now applied an estimation method that includes ambient total daily photosynthetic available irradiation (PAR; J m-2 d-1).
If ambient production (Pz; mgC m-3 h-1) is linearly related with ambient light intensity (Iz; J m-2 h-1), the production at a certain depth (z; m) can be described according to:
Pz = B · a · IzPAR
where B (mgChla m-3) is the phytoplankton biomass, and a is the biomass-specific production rate (mgC m-3 h-1 (mgChla m-3)-1 (J m-2 h-1) -1). Light intensity (IzPAR; J m-2 h-1) at a certain depth (z; m) can be described according to:
IzPAR = I0PAR e –kz
where I0PAR (J m-2 h-1) is the photosynthetic available irradiation just under the water surface, and k (m-1) is the attenuation coefficient. Consequently, the integrated production in the water column (PK; mgC m-2 h-1), up to depth d (m), can be described according to:
PK = B · a · k-1 · I0PAR · (1 - e-kd)
For turbid waters, where the value for e-kd is very small, the in situ daily primary production (Pin situ; mgC m-2 d-1) can be estimated according to:
Pin situ = B · a · k-1 · L · I0PAR daily sum
where I0PAR daily sum (J m-2) is the daily-summed photosynthetic available irradiation during daylight hours.
Daily summed total irradiation was daily measured at a weather station of the Royal Dutch Meteorological Organisation (KNMI) in Den Helder, located approximately 5 km south of the phytoplankton sampling station (KNMI ISSN 0167-8248). We applied a fixed term of 7% for daylight reflection at the water surface (Højerslev 1978), and assumed that photosynthetic available radiation (PAR) accounted for 50% of the total irradiation as measured at the weather station.
We estimated the value of the factor [B·a] by assuming that the production-light curve is roughly linear over the range of relevant light intensities in the water column, as well as over the range from 0 to light intensity during incubation (Iincub). Earlier publications indicated that light intensity in the laboratory during incubation was approximately 0.1 cal cm-2 min-1 (Cadée and Hegeman 1974), which equals 70 W m-2. Recent measurements by means of a spherical QLS-100/101 quantum scalar irradiance meter showed that this light intensity was 28 J cm-2 h-1.For phytoplankton communities in shallow coastal waters, the irradiation at the onset of saturation (EK) varies between 200 and 500 µE m-2 s-1 (Kirk 1996), which equals 44 to 109 W m-2 (for λ = 550 nm), corroborating that light conditions during incubation were below light saturation levels for phytoplankton growth. Under those assumptions:
B · a = Pincub / IincubPAR
where Pincub (mgC m-3 h-1) is the primary production during incubation at ambient temperature and constant light conditions (J m-2 h-1), from which:
Pin situ = Pincub· IincubPAR-1· k-1 · I0PAR daily sum · 0.93
The value of the attenuation coefficient k (m-1) was derived from the ambient Secchi depth (dSecchi ; m) according to k = 1.9596 dSecchi-0.9424. This empirical relationship (r2 = 0.92) was based on a combined data set (n = 505) of light intensities in the Eastern and Western Scheldt, The Netherlands, derived from Wetsteyn and others (2003) and from Kromkamp Peen (unpublished).