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Relationship Between Nutrient Enrichment and Benthic Function: Local Effects and Spatial Patterns
Citation:
ABDELRHMAN, M. AND G. CICCHETTI. Relationship Between Nutrient Enrichment and Benthic Function: Local Effects and Spatial Patterns. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 35(1):47-59, (2012).
Impact/Purpose:
The purpose of this work is to provide a methodology to analyze effects of nutrient enrichment on benthic community health using sediment profile imaging. The methodology is based on studying local effects within a system rather than the whole system. It successfully identified the stressor-response relationship between the spatial distribution of long-term average concentration of nitrogen and the structure and function of fauna in a number of embayments. Results from northeastern USA estuaries showed a maximum bioturbation rate at intermediate nitrogen concentrations, or a “hump-backed” pattern of response. When concentration expands the full range (from low to high values) within a system the relevant spatial pattern of faunal response peaks at medium concentrations in the central part of the embayment and it drops toward the inner and outer regions where concentrations are usually high and low, respectively, as regulated by tidal flushing. There are many advantages to the presented approach. First, it is applicable to any embayment regardless of its geographical location, which widens the spectrum for cross-comparisons between various systems. Also, if nutrient loads change in time (e.g. due to changes in land uses) a respective analysis can be easily produced to identify the new spatial distribution of local average concentrations and predict the corresponding levels of faunal activities. In addition, this approach highlights the importance of considering local effects and spatial patterns in management of nutrient effects and identifying hotspots in coastal systems.
Description:
Eutrophication-induced changes to benthic structure and function are problems of enormous ecological and economic significance. Understanding the relationships between nutrient enrichment and effects, modifying factors such as localized transport time, and symptoms of eutrophication such as altered benthic function is very important to managing coastal systems. Because many management questions are location-specific, we explored the extent to which spatial patterns both within and among estuaries affect ecological response. Nutrient exposures were characterized along estuarine gradients using a parameter that integrates long-term average nitrogen concentrations and exposure periods. This was related to the depth of the apparent Redox Potential Discontinuity (aRPD, the oxidized sediment layer), quantified from sediment profile images as a surrogate indicator of bioturbation. Our results, from 22 northeastern USA estuaries, showed a maximum bioturbation rate at intermediate nitrogen concentrations, or a “hump-shaped” pattern of response. This is consistent with the findings of others, that initial enrichment levels lead to enhanced benthic biomass and activity under conditions of food limitation, while continued enrichment leads to declines in benthic biomass and function under oxygen stress. Our results suggest that the benthic functional response to nutrient input ─ increasing and then decreasing levels of bioturbation ─ is present among the complex spatial patterns within and among northeast estuaries in the USA, when localized nutrient effects are evaluated in a spatial context. We suggest that local effects and spatial patterns should be considered when managing nutrient effects in these and other systems.