Detrimental effects of sedimentation on marine benthos: what can be learned from natural processes and rates?

Abstract

Benthic organisms are adapted to the natural processes of sediment movement, erosion and deposition. Laboratory studies have cataloged the range of responses to flow and sediment movement that allow benthos to survive, and even to thrive, under intense, storm-driven sediment movement. Extreme sedimentation events also result from man's modifications of the nearshore marine environment, and the scale and magnitude of these alterations can often greatly exceed that of natural occurrences. Unfortunately, there is little of the quantitative information necessary for predicting how materials placement, sediment deposition and erosion will affect the ecology of these environments. We are using both field and laboratory approaches in Delaware Bay to address two questions. First, what rates and frequencies of sediment movement characterize natural events, and second, what rates and frequencies are detrimental to representative benthic species and functional groups. We present these results as case studies that address ecological impacts of dredge materials placement, site selection and benthic community responses. Quantifying natural sedimentation rates and the susceptibility of macrofauna by functional groups are both critical to reliably predicting environmental impacts. If biological effects are parameterized appropriately (i.e. in terms of natural processes), it may be possible to employ the existing knowledge-base of benthic ecology to predict effects of disturbances and to design projects that will minimize these impacts. Materials placement that is analogous to natural events should allow community responses to follow natural seasonal and successional trends and to exhibit minimal anthropogenic impacts. When sedimentation exceeds natural thresholds, then impacts may involve total loss of the community and subsequent colonization by pioneer species. In this latter case, an entirely different suite of ecological processes will drive impacts and recovery, potentially leading to dramatically altered benthic communities. Understanding organisms’ sublethal responses and drawing on experimental ecological studies will lead to improved prediction of benthic community responses and more reliable assessment of project impacts.