Artificial structures as a source of elevated detrital loads for sedimentary environments

Abstract

Artificial structures, such as seawalls and breakwaters, are essential components of shoreline protection, yet they can have major consequences for marine organisms, particularly those in surrounding sediments. Improved understanding of the mechanisms by which artificial structures influence sedimentary habitats and the scale at which these mechanisms operate is needed to assess impacts of intensifying shoreline armoring on nearshore ecosystems. We examined detrital production on artificial structures and influx into surrounding sedimentary habitat in Puget Sound, Washington. Benthic surveys of hard substrates and drift composition indicated that red macroalgae and encrusting invertebrates were the major producers of detrital material on subtidal artificial structures in the region. Core samples collected at three distances (1, 7, and 15 m) from selected artificial structures revealed that these materials were moving into adjacent sedimentary seascapes and becoming incorporated into the upper layers of sediment. Though detrital influx occurred only in the immediate vicinity of artificial structures, it altered sediment characteristics considerably, increasing the quantity of shell fragments and thus the average grain size, as well as the concentration of red macroalgal detritus in sediments. Furthermore, rapid turnover rates of macroalgal detritus in experimental sediment plots suggested the influx of red macroalgal material from artificial structures was a frequent occurrence. These results indicate that elevated detrital influx is a persistent process in habitats bordering artificial structures in Puget Sound. Further work is needed to assess ecosystem response to detrital influx and to understand detritus-related impacts of shoreline armoring at the landscape scale.