Nutrient limitation at the ecosystem and the phytoplankton community level in the Laholm Bay, south-east Kattegat

Nutrient enrichment experiments in the laboratory with surface water from the Laholm Bay, indicate that phytoplankton is nitrogen-limited. Phosphate addition to phytoplankton samples rarely caused a biomass increase in agreement with negligible alkaline phosphatase activity. Usually the inorganic-N/P quotient in surface water was also below the Redfield ratio (16:1, by atoms), indicating potential nitrogen limitation. The supply ratio (runoff and deep water) of inorganic nutrients to the surface water of the bay was 23N:1P for March–October and thus indicates a mean shortage of phosphate. During summer when river runoff is small, the low N/P-ratio in the deep water (10:1) which is mixed into surface water suggests nitrogen limitation, in agreement with the enrichment experiments. Benthic denitrification and the low N/P-ratio of Zooplankton excretion may make regenerated primary production nitrogen-limited. The recent suggestion by Smith (1984), that marine ecosystems, like lakes, are phosphorus-limited because any nitrogen shortage is alleviated through nitrogen fixation, does not apply to the bay itself, where water exchange is too rapid to allow for an adjustment of the N/P-supply quotient.