Primary production of benthic microalgae in two shallow coastal lagoons of different trophic status in the southern Baltic Sea

Abstract

Primary production of undisturbed sediments, taken from 2 shallow coastal lagoons in the southern Baltic Sea, was measured in situ and in the laboratory. In the highly eutrophicated and turbid water (mean attenuation coefficient = 2.97 m-1) of Kirr-Bucht (Darss-Zingster Bodden), chlorophyll a concentrations were about 8 times higher than at moderately eutrophicated (mean attenuation coefficient = 0.66 m-1) Rassower Strom (Nordrugensche Bodden). The difference between the 2 areas in terms of chlorophyll a concentrations of the sediment was smaller (only about 2.5 times higher at Kirr-Bucht). Sediment oxygen consumption rates were similar at both sampling stations, showing a marked seasonal trend. Hourly rates of gross production, measured by in situ incubations, revealed maximal values of 30.0 and 17.6 mg C m-2 h-1 for sediments of Kirr-Bucht and Rassower Strom, respectively. Agreement between production rates measured in situ and in laboratory incubations was acceptable (R2 = 0.833). Irradiance dependency of gross production could be well described by 2 parameters, α (slope of light-saturation curve at low irradiance) and P(m) (photosynthetic capacity), of a photosynthesis model. Calculated annual rates for Kirr-Bucht (0.6 m water depth) were 60 g C m-2 yr-1 for gross production and 5 g C m-2 yr-1 for net production. For Rassower Strom (3:4 m water depth), 23 g C m-2 yr-1 for gross production and -31 g C m-2 yr-1 for net production were calculated. If production rates were extrapolated to irradiance conditions at comparable water depths at both sampling locations (e.g. 1 m), the detrimental effect of the eutrophicated water column of Kirr-Bucht on benthic primary production became evident in the gross production rates (Kirr-Bucht: 25 g C m-2 yr-1, Rassower Strom: 51 g C m-2 yr-1) and was even more significant for the net production rates (Kirr-Bucht: -30 g C m-2 yr-1, Rassower Strom: -3 g C m-2 yr-1). At a depth of 1 m, a much higher deficit in the carbon balance for the benthic system of Kirr-Bucht was calculated than for Rassower Strom. Therefore, benthic community respiration at the highly eutrophicated location could not by a large margin be balanced by authochthonous benthic primary production, whereas, at the moderately eutrophicated location, benthic respiration and production were nearly balanced.