Relationships between Nutrient Enrichment and Benthic Function: Local Effects and Spatial Patterns

Abstract

Eutrophication-induced changes to benthic faunal activities are problems of significant ecological impact, affecting global nutrient budgets as well as local trophic connections. We address the question of how nitrogen loads to estuarine embayments alter the bioturbation activities of benthic fauna. Specifically, we related local benthic activities to calculated local nitrogen concentrations for 22 northeastern US estuaries. These local nitrogen concentrations were derived from the calculated nitrogen loading for the embayment together with the spatial distribution of the local flushing time. Our results showed a maximum bioturbation rate at intermediate nitrogen concentrations or a "hump-shaped" pattern of response. This behavior was evident in all embayments that had a range of concentrations including low, intermediate, and high values. Embayments where sampling did not include this full range did not show this behavior. This work provides methods and guidance to help managers make decisions concerning the effects of nitrogen loading on the activities and wellbeing of benthic fauna in coastal embayments. The novelty of this approach lies in identifying the response of bioturbation to nitrogen loading in many systems, without costly and time-consuming speciation of benthic fauna, and also in rapidly identifying embayments and aquatic areas with vulnerable fauna. These results are ecologically significant in supporting the hypothesis that benthic organism abundance and activity will peak at mid-levels of nitrogen due to the interplay of food availability and oxygen levels, noting that the critical levels of these factors differ among water bodies.

Response of a marine benthic invertebrate community and biotic indices to organic enrichment from sewage disposal

“…Species richness may show a range of responses to stress or resource availability including the humpbacked curve described by the intermediate-disturbance hypothesis, which shows an increase in diversity with increasing stress or resource availability before decreasing again as stress continues to increase (Connell, 1978;Odum, 1985;Dodson et al, 2000;Mittelbach et al, 2001;Hooper et al, 2005;Huston, 2014). This pattern has also been shown for the response to nitrogen loadings in functioning of benthic invertebrate communities, for example in bioturbation rates (Abdelrhman & Cicchetti, 2012). In some cases, nutrient enrichment can increase species richness and productivity through increased survival and recruitment, by making more resources available and mitigating the effects of other stressors such as heavy metal contaminants (Lawes et al, 2016).…”

“…This response was in line with studies that have found nutrient enrichment can lead to increased species richness, greater availability of resources, greater productivity and rates of functioning and increased survival and recruitment in benthic communities (e.g. Borja et al ., 2009 b ; Abdelrhman & Cicchetti, 2012; Krumhansl et al ., 2015; Lawes et al ., 2016). Species richness can respond to disturbance by initially increasing, corresponding to the intermediate disturbance hypothesis (Connell, 1978; Odum, 1985; Dodson et al ., 2000; Mittelbach et al ., 2001; Hooper et al ., 2005; Huston, 2014).…”

“…However, input of waste into coastal areas can cause changes to benthic communities (e.g. Stull et al ., 1986; Bowen & Valiela, 2001; Diaz & Rosenberg, 2008; Abdelrhman & Cicchetti, 2012; Caswell et al ., 2018). Eutrophication with deoxygenation due to organic enrichment, can lead to lower benthic species richness; degraded ecosystem functioning, such as bioturbation and nutrient cycling; and decreased capacity to provide ecosystem services, including waste treatment, and climate regulation through reduced carbon sequestration (Worm et al ., 2006; Smith & Schindler, 2009; Caswell et al ., 2018).…”