Beyond beach width: Steps toward identifying and integrating ecological envelopes with geomorphic features and datums for sandy beach ecosystems

Abstract

Our understanding of ecological responses to climatic and anthropogenic forcing lags far behind that of physical or geomorphic responses for beach ecosystems. Reconciling geomorphic features of beaches with ecological features, such as intertidal zones and mobile biota that are not described by beach width alone, could help address this issue. First, although intertidal zones characterized by distinct groups of mobile burrowing animals are described for beaches, the locations and elevations of these zones do not coincide with standard shoreline datums. Second, intertidal zonation on beaches is extremely dynamic due to the combination of unstable sandy substrate and a highly mobile biota; shifting strongly with tides, waves, storms, and beach conditions. We propose that beach biota use ecological “envelopes” of cross-shore habitat to cope with constantly changing beach conditions. We estimated the extent of these “envelopes” for a variety of taxa on tidal to daily, semi-lunar and seasonal to annual time scales, using literature values on cross-shore animal movements and a field study of the positions of intertidal beds of two species of typical mid and upper shore beach invertebrates. Daily or tidal cross-shore movement varied most (1m to 100m) with daily “envelopes” covering 7% to 85% of the available beach width. Semi-lunar movement (12m) and envelopes (28%) were relatively small, while estimated annual “envelopes” were large, averaging 61% of beach width. The large scope of annual ecological envelopes relative to beach widths reflects how intertidal animals escape seasonally extreme or episodically harsh conditions. Intertidal bed positions of a talitrid amphipod and an opheliid polychaete correlated well with selected beach features in our field study suggesting that incorporation of ecological envelopes in models of shoreline evolution may be feasible. Describing ecological zones in terms of more dynamic shoreline features, such as total water level (TWL) that incorporate wave setup and runup, may be particularly applicable to upper intertidal biota whose distributions closely followed the high tide strand line (HTS), a feature which tracks total water level (TWL). Developing a TWL approach may also provide new insights on habitat availability for beach nesting wildlife and coastal strand vegetation. Conservation of beach ecosystems could be enhanced by incorporating sufficient beach habitat to accommodate the dynamic ecological envelopes used by mobile intertidal invertebrates and wildlife.