The legacy effect of changing nutrient inputs to coastal ecosystems

Nutrient management plans have been successful in reducing nutrient inputs to many coastal ecosystems, but ecosystem responses have been unanticipatedly weak. This lack of recovery has been attributed to a legacy effect of past nutrient inputs, possibly sustaining sediment nutrient release and eutrophication over longer periods. We analyzed sediment pools of carbon, nitrogen and phosphorus sampled over 25 years (1999-2023) across 27 Danish estuaries, following substantial reductions in inputs of nitrogen (>50%) and phosphorus (>90%) from land, the majority of these occurring from 1985 to 1997. Variability among sediment cores was high, both on spatial and temporal scale, although it was reduced by normalizing total N and P to loss of ignition. All sediment pools changed significantly with depth, but there was no significant difference among estuaries despite large differences in area-specific nutrient loading, highlighting the large spatial variability within estuaries. No significant changes in sediment pools were observed over the study period, with an almost even distribution of directions of change. Given the relatively large sampling effort (>100 cores), we estimated that it should be possible to detect changes of 20% with a probability of 80%. Combining this with the lack of consistent trends suggests that the legacy effect of nutrient reductions was within a few years rather than decades or that the legacy effect is small. Hence, the lack of coastal ecosystem recovery is most likely due to other factors.

Presentation Preference: Oral

Primary Presenter: Jacob Carstensen, Aarhus University (jac@ecos.au.dk)

Authors:

Signe Høgslund, Aarhus University, Ecoscience  (sigh@ecos.au.dk)

Jeremy Testa, University of Maryland, Chesapeake Bay Laboratory  (jtesta@umces.edu)