Beyond beach width - integrating ecological zones and function of sandy beach ecosystems

Sandy beaches receive large subsidies of drift kelp and macrophytes from kelp forests.  Rapid processing of this macroalgal wrack by intertidal invertebrates (shredders) and its subsequent decomposition and remineralization can lead to high concentrations of dissolved nitrogen in intertidal pore water (>1000 uM, primarily nitrate). Our ongoing research seeks to understand how beaches function as 1) recipient and processing ecosystems for large subsidies of organic matter from kelp forests 2) filters of seawater and groundwater and 3) sources of regenerated nutrients to the coastal ocean and nearshore primary producers. When dryer upper beach zones are narrow or absent, macrophyte wrack accumulation is significantly reduced on SBC beaches. Likewise, armored beaches had significantly narrower upper and mid beach zones and lower standing stocks of wrack (10x-1000x less) than adjacent unarmored beaches. Our studies of the functional role of upper intertidal zones in macrophyte wrack accumulation, consumer abundance and nutrient cycling using a gradient design along ~ 1 km of bluff-backed beach suggest width of the upper sandy intertidal zone is a key ecological feature.  Although width of the upper beach zone varied < 20m across the study area, wrack biomass and consumer abundance varied >1000 fold and were significantly correlated with upper intertidal width as were concentrations of dissolved inorganic nitrogen in intertidal pore water. Variation in carbon stable isotope signatures of intertidal POM and a deposit-feeding polychaete suggested that dietary sources shifted from phytoplankton to a mixture of wrack and phytoplankton as the upper beach zone expanded and wrack retention increased. Our results suggest even relatively narrow upper sandy intertidal zones are critical for maintaining organic matter processing and nutrient regeneration   in beach ecosystems. Climate-induced sea level rise, which is expected to increase coastal erosion and expand the use of coastal armoring, will reduce beach widths and  strongly alter these ecological functions.