Shoot morphometry and production dynamics of eelgrass in the northern Baltic Sea

Abstract

Zostera marina L. related parameters and environmental variables (hydrography and sediment porewater nutrients) were measured from May to October, 1998 at two sites in the northern Baltic Sea (61°N and 20°E). Density (70–400 shoots m −2 ), biomass and production were low. Biomass peaked in August (20 g dry weight (dw) m −2 , range: 4–33 g dw m −2 ). Production displayed seasonal and vertical patterns, occasionally correlating with temperature, but differing between the sites. At site 1, production was similar at shallow and intermediate depths (average: 75 and 85 mg dw m −2 per day, respectively) and constantly higher at the lower (4.2 m) boundary (average: 130 mg dw m −2 per day). At site 2, the highest production (average: 370 mg dw m −2 per day) was recorded at the upper (1.4 m) distribution limit, while production at the lower boundary (3.9 m) was constantly lower (250–80 mg dw m −2 per day). In a global comparison, these values are the lowest recorded for Z. marina , most likely because of light limitation (deep stations only) and low nutrient concentrations in both the water column and in the sediment. The results suggest, that Z. marina , occurring only at exposed, organically poor, sandy sediments in the northern Baltic Sea, may temporarily overcome nutrient limitation by utilising elevated nutrient levels from land run-off, decaying algal masses and/or upwelling events. These mechanisms most likely explain the observed differences between the sites studied.