Spatial and interannual variations with depth in eelgrass populations

Abstract

Measurements (1400) of eelgrass ( Zostera marina L.) shoot density and biomass along 19 depth transects in Øresund between Denmark and Sweden were analyzed during late summer in 4 years to characterize the spatial and temporal variation in shallow, intermediate and deep waters. The reduced physical harshness and reduced light availability with depth led to the hypotheses that: (1) variability in shoot density and biomass with depth is highest in shallow perturbed waters and in deep light-limited water and lowest at intermediate depths, and (2) spatial and temporal variabilities resemble each other because they are influenced by the same underlying causes. Spatial variances in both shoot density and biomass were significantly positively related to mean values at all depths and variances, therefore had to be compared based on relationships between variances and mean values. The hypotheses were supported by the findings that spatial and temporal variances in shoot density were both significantly highest in shallow water where high physical disturbance and high irradiance are conducive to high variability. However, we observed no significant differences in the variability of shoot density between intermediate and deep waters suggesting that the effects of reduced disturbance and the increased risk of severe light limitation at the depth boundary outweighed each other. Spatial variance in biomass did increase more with means in deep water than in shallow and intermediate waters, while temporal variance showed no differences with depth. Overall, spatio-temporal variability in shoot density and biomass relative to means declined at higher mean values probably because self-shading and space limitation set an upper boundary on eelgrass abundance. Wrong conclusions would have emerged if variances or coefficients of variation of shoot density and biomass with depth had been compared directly without correcting for the influence of different means.