Sandy beaches comprise the majority of coastlines globally and support diverse ecosystems within a narrow intertidal habitat. Humans are increasingly inhabiting coastal regions and exposing these ecosystems to the effects of anthropogenic activities such as dredging, fishing, and recreation. Morro Bay, located within San Luis Obispo County on California’s central coast, is home to sandy beach and estuarine ecosystems as well as an economically valuable fishing industry and a natural protected harbor. In February 2017 the U.S. Army Corps of Engineers dredged the harbor’s navigational channel to remove 240,000 cubic yards of accumulated sediment, which was deposited two kilometers north for beach nourishment. While local-scale processes (i.e. dredging) have been generally shown to impact beach ecosystems, we address the extent of these impacts specifically on Morro Bay beach biodiversity. In July 2017 we initiated a year-long study involving monthly bird counts and quarterly invertebrate counts at seven survey sites, including the site of deposition, along the Morro Bay coast. From this census data, we calculate quantitative biodiversity values as determined by Simpson’s and Shannon’s diversity indices for the purpose of evaluating spatio-temporal patterns of biodiversity and any significant impacts on biodiversity correlated with the deposition of dredge material.
Results/Conclusions
If the deposition of dredged sediment is negatively correlated with bird and invertebrate diversity, we expect to find the lowest diversity at the site of deposition and the highest diversity at the survey sites furthest North and South of the deposition. Bird diversity calculations yielded values ranging from 1.09 to 1.66 (Shannon’s diversity) and 0.57 to 0.74 (Simpson’s diversity) for the control sites North and South of the site of deposition. At the site of deposition, calculations yielded values of 1.31 (Shannon’s diversity) and 0.64 (Simpson’s diversity), which fall at the median of the diversity value distributions for both metrics. Invertebrate diversity calculations showed similar patterns for both metrics. Preliminary statistical analyses using both diversity metrics for birds and invertebrates yield no significant differences and thus do not reflect a pattern of lower beach biodiversity at the nourishment site. Continued sampling and an analysis of variance of means across months and sites will allow clearer determination of spatial and temporal trends of diversity. Our study will provide a greater understanding of the effects of depositing dredged sediment, which can help inform future decisions regarding potentially ecologically impactful projects in Morro Bay and elsewhere along the California coast.