Abstract
Effects of warming and nutrient enrichment on intact unvegetated shallow-water sediment were investigated for 5 weeks in the autumn under simulated natural field conditions, with a main focus on trophic state and benthic nitrogen cycling. In a flow-through system, sediment was exposed to either seawater at ambient temperature or seawater heated 4°C above ambient, with either natural or nutrient enriched water. Sediment–water fluxes of oxygen and inorganic nutrients, nitrogen mineralization, and denitrification were measured. Warming resulted in an earlier shift to net heterotrophy due to increased community respiration; primary production was not affected by temperature but (slightly) by nutrient enrichment. The heterotrophic state was, however, not further strengthened by warming, but was rather weakened, probably because increased mineralization induced a shortage of labile organic matter. Climate-related warming of seawater during autumn could therefore, in contrast to previous predictions, induce shorter but more intensive heterotrophic periods in shallow-water sediments, followed by longer autotrophic periods. Increased nitrogen mineralization and subsequent effluxes of ammonium during warming suggested a preferential response of organisms driving nitrogen mineralization when compared to sinks of ammonium such as nitrification and algal assimilation. Warming and nutrient enrichment resulted in non-additive effects on nitrogen mineralization and denitrification (synergism), as well as on benthic fluxes of phosphate (antagonism). The mode of interaction appears to be related to the trophic level of the organisms that are the main drivers of the affected processes. Despite the weak response of benthic microalgae to both warming and nutrient enrichment, the assimilation of nitrogen by microalgae was similar in magnitude to rates of nitrogen mineralization. This implies a sustained filter function and retention capacity of nutrients by the sediment.
Highlights
With an ongoing environmental change, the structure and function of aquatic ecosystems are challenged, by local anthropogenic stressors such as eutrophication, toxicants and physical disturbance, and by global stressors such as increased seawater temperature and acidification [1,2]
Previous experiments have shown that the net response of natural, multi-trophic sediment systems to various natural and anthropogenic stressors often change over time due to the simultaneous operation of both direct and indirect effects [38,52,53]
Warming pushed the system back towards autotrophy at the end of the experiment (Figure 2G, 2H). (2) Rates and pathways of nitrogen cycling were affected by both warming and nutrient enrichment, supporting our second hypothesis
Summary
With an ongoing environmental change, the structure and function of aquatic ecosystems are challenged, by local anthropogenic stressors such as eutrophication, toxicants and physical disturbance, and by global stressors such as increased seawater temperature and acidification [1,2]. It has been suggested that future warming will alter food-web structure and ecosystem functioning [4,5] and exacerbate the effect of other stressors, such as ocean acidification and toxicants [6,7]. It is still difficult to draw any general conclusions about the importance of various types of interactions (synergistic, antagonistic or additive) in coastal marine ecosystems [2]. This is true for shallow-water sediment systems [2,11]
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