EFFECTS OF BEACH REPLENISHMENT ON THE NEARSHORE 

 SAND FAUNA AT IMPERIAL BEACH, CALIFORNIA 



by 



Tevence VccoVy Douglas Bienev 



and Stephen Lacy 



I . INTRODUCTION 



Unconsolidated sediments comprise the most prevalent nearshore 

 benthic marine habitat. Biological communities associated with these 

 sediments assimilate much of the energy flow through the nearshore 

 ecosystem. Soft-bottom invertebrates play an integral role in the 

 conversion of the organic deposits of the continental shelves into 

 biomass available to higher trophic levels such as predatory demersal 

 fishes whose population densities show positive correlation with benthic 

 invertebrate standing crops (Stevens, 1930; Longhurst, 1958; Day, 1967). 



This report presents results from a study of impacted and poten- 

 tially impacted sedimentary communities in and near an area where approxi- 

 mately 765,000 cubic meters of dredged sediment was pumped onto a coastal, 

 exposed beach to replenish part of the shoreline at Imperial Beach, 

 California. This process may also be referred to as beach nourishment 

 (Allen, 1972). The aim of the study was to establish relationships be- 

 tween beach replenishment and measurable biological variables in the 

 shallow-water community (e.g., composition, species abundances, and di- 

 versity) and those measurable abiotic variables (e.g., sediment type) 

 considered important for their influence on biological community struc- 

 ture. A review of general ecological effects associated with dredging 

 and beach replenishment was presented by Thompson (1973) . 



The open-coast shoreline of southern California is about 84 percent 

 sand beach. Of the beaches, only 30 percent are truly depositional 

 regions of the coast (Emery, I960) . Terrestrial inputs of sediment to 

 southern California beaches have been reduced as a result of man's 

 activities and serious beach erosion has been a problem for some time at 

 various places along the coastline. Beach replenishment with dredge 

 material from depositional environments such as bays provides a feasible 

 means of counteracting beach erosion in certain areas. This study 

 discusses the biological effects associated with this process. 



The program design subscribed to sampling at points (space, time) 

 which could test various hypotheses related to effects of the added 

 sediments. Although correlations between physical-chemical and bio- 

 logical events may be observed, the idea of causation is a complex one 

 and correlated variables may not always be directly related. To fully 

 understand mechanisms underlying observed distributions, the detailed 

 relationships of species associations to environmental parameters may 

 require subsequent experimental verification. Jumars and Fauchald 

 (1977) emphasize the importance to sedimentary communities of factors 



