TY - JOUR
T1 - Reduced cover of drifting macroalgae following nutrient reduction in Danish coastald waters
AU - Rasmussen, Jonas Ribergaard
AU - Dromph, Karsten Mikael
AU - Göke, Cordula
AU - Krause-Jensen, Dorte
PY - 2015
Y1 - 2015
N2 - Based on a large dataset from the national Danish monitoring programme, we analysed the temporal variability of drifting algae cover in shallow (1–2 m) water during a period of reduced nutrient loadings. Algal cover was analysed both in absolute terms and relative to eelgrass, Zostera marina, cover to test the hypotheses that (1) the cover of drifting algae and the relative dominance of algae versus eelgrass in shallow waters have declined in parallel to reductions in nutrient levels during the last decades, and (2) spatio-temporal differences in algal cover can be related to differences in nutrient conditions and environmental characteristics. The cover of drifting algae was positively related to total nitrogen concentration and Secchi depth but negatively related to exposure, salinity and mean summer temperature. The cover of drifting macroalgae showed significant declines over the past two decades, paralleling the reduction in nutrient concentrations. The present cover of drifting algae is low (<10 %) and probably pose little threat to the general distribution of eelgrass in shallow Danish waters though local accumulations may still be harmful. However, the ratio between drifting algae cover and eelgrass cover showed no significant trend, reflecting that eelgrass cover had not increased despite the reduced levels of nutrients and drifting algae. This ratio also showed no consistent relationship to water quality probably because different regulation mechanisms govern drifting algae and eelgrass, and feedback mechanisms may delay eelgrass recovery. Reduced drift algal cover may be an early sign of improved ecological status, while further improvements in terms of recovery of eelgrass meadows have longer perspectives
AB - Based on a large dataset from the national Danish monitoring programme, we analysed the temporal variability of drifting algae cover in shallow (1–2 m) water during a period of reduced nutrient loadings. Algal cover was analysed both in absolute terms and relative to eelgrass, Zostera marina, cover to test the hypotheses that (1) the cover of drifting algae and the relative dominance of algae versus eelgrass in shallow waters have declined in parallel to reductions in nutrient levels during the last decades, and (2) spatio-temporal differences in algal cover can be related to differences in nutrient conditions and environmental characteristics. The cover of drifting algae was positively related to total nitrogen concentration and Secchi depth but negatively related to exposure, salinity and mean summer temperature. The cover of drifting macroalgae showed significant declines over the past two decades, paralleling the reduction in nutrient concentrations. The present cover of drifting algae is low (<10 %) and probably pose little threat to the general distribution of eelgrass in shallow Danish waters though local accumulations may still be harmful. However, the ratio between drifting algae cover and eelgrass cover showed no significant trend, reflecting that eelgrass cover had not increased despite the reduced levels of nutrients and drifting algae. This ratio also showed no consistent relationship to water quality probably because different regulation mechanisms govern drifting algae and eelgrass, and feedback mechanisms may delay eelgrass recovery. Reduced drift algal cover may be an early sign of improved ecological status, while further improvements in terms of recovery of eelgrass meadows have longer perspectives
KW - drift algae
KW - eelgrass
KW - Zostera marina
KW - nitrogen
KW - nutrients
KW - relative exposure index
U2 - 10.1007/s12237-014-9904-4
DO - 10.1007/s12237-014-9904-4
M3 - Journal article
SN - 1559-2723
VL - 38
SP - 1664
EP - 1677
JO - Estuaries and Coasts
JF - Estuaries and Coasts
IS - 5
ER -