Linking the abundance of mobile aquatic macrofauna to land cover and shoreline alteration in coastal estuarine habitats

Background/Question/Methods

Watershed land cover is often associated with landscape-scale patterns in freshwater biological communities, but examples from coastal marine systems are limited.  As human populations expand, there is also growing interest in the ecological consequences of replacing natural shoreline habitat with hardened structures.  We examined both watershed land cover and shoreline alteration as predictors of community composition and abundance of nearshore aquatic macrofauna (fish and crabs) in Chesapeake Bay, the largest estuary in the United States.   Data on 22 commonly occurring nearshore species were compiled from 648 sites spanning the entire estuary.  To allow for comparison across land cover gradients, data were collected from 45 different subestuaries.  Land cover around each subestuary was assessed at the whole watershed scale and within a 100m buffer from shore.  Generalized linear mixed models were used to assess patterns in species specific abundance across subestuaries and habitats while accounting for the multilevel structure of the data (subestuary and site scales).  In addition, we focused on the effects of shoreline alteration within a subset of 16 subestuaries that were deliberately sampled at natural (beach, marsh) and altered (riprap, bulkhead) shorelines. Redundancy analysis was used to evaluate how shoreline habitat affected community structure.  Salinity, an important covariate, was accounted for in all models.

Results/Conclusions

There were multiple significant correlations between species-specific abundance and land cover, even when including shoreline habitat and salinity in statistical models.  Patterns with land cover often emerged independent of salinity, but sometimes patterns were only observed over a specific salinity range (either 0-10 PSU or >10 PSU).  Generally, species abundance patterns followed established negative relationships between human land cover and water quality.  For example, there were significant negative relationships between cropland and the abundance of blue crabs and Atlantic silversides, and between developed land and the abundance of several species.  Conversely, there were significant positive relationships between wetland within 100m of shore and abundance of multiple species including blue crab.  At local scales, shoreline habitat explained 24.4% of the variation in macrofauna community structure within 3m from shore and 16.7% of the variation within 16m from shore.   Small littoral fishes were strongly associated with marshes, while pelagic/planktivorous fishes were strongly associated with beaches.  Larger bodied species were often encountered near the land/water interface at ripraps and bulkheads, which lacked crucial shallow water refuge habitat.  By identifying patterns in mobile, marine species, these findings advance our understanding regarding the ecological effects of human-related activities.