Effects of local shoreline and subestuary watershed condition on waterbird use: influences of geography, scale, and season in the Chesapeake Bay

In many coastal regions throughout the world, there is increasing pressure to harden shorelines to protect human infrastructures against sea-level rise, storm surge and erosion. This study examines waterbird community integrity in relation to shoreline hardening and land use characteristics at three geographic scales: (1) the subestuary scale characterized by seven shoreline types: bulkhead, riprap, developed, natural marsh, Phragmites-dominated marsh, sandy beach and forest; (2) the 500m landscape scale surrounding the shoreline edge; and (3) the watershed scale. From 2010 to 2014, we surveyed waterbirds within 21 subestuaries throughout the Chesapeake Bay during two seasons to encompass influx of migrating shorebirds and wading birds in late summer and migrating and wintering waterfowl in late fall. We used an Index of Waterbird Community Integrity (IWCI) based on sensitivity to human disturbance to characterize the waterbird community for each subestuary and season. IWCI scores ranged from 14.3 to 19.7. Multivariate regression model selection showed that the local subestuary scale had the strongest influence on IWCI. At this scale, percent bulkhead and Phragmites within a subestuary were the strongest predictors of IWCI, with a 10% increase in either resulting in a decrease of 0.4 to 0.5 IWCI units. Recursive partitioning revealed that thresholds greater than approximately 5 or 10% (Phragmites, bulkhead) led to a decrease in waterbird community integrity. Our results indicate that shoreline development at the subestuary scale has an important effect on waterbird community integrity, and that shoreline hardening and invasive Phragmites have a negative effect on waterbirds using subestuarine systems.