Patterns and processes affecting the transport, retention and fate of trophic subsidies to sandy beach ecosystems

Background/Question/Methods: Understanding dynamics of trophic connections between ecosystems is necessary for predicting how food webs may respond to environmental change. Our study quantifies trophic connectivity between a highly productive donor ecosystem, kelp forests, and a recipient ecosystem with low primary production, sandy beaches. Kelp forests export large amounts of drift kelp that deposit on sandy beaches where these subsidies fuel a diverse and productive intertidal food web. To evaluate how the dynamics of trophic connectivity to kelp forests influences beach ecosystems, we are quantifying physical transport and input of drift kelp from kelp forests to sandy beaches. We are tracking kelp fate using results from three kelp forests and along 25 km of adjacent coastline in the Santa Barbara Channel, California. We observed fate and transport of kelp using complimentary approaches: 1) tracking kelp plants tagged at three focal kelp forests using GPS drifters; 2) tagging large numbers of kelp plants with “drift cards” for recovery by the project team and the public on beaches and 3) conducting monthly counts of kelp plants deposited along 25 km of shoreline.

Results/Conclusions: Our results characterizing patterns of transport of giant kelp (Macrocystis pyrifera) exported from kelp forests indicate high spatial and temporal variability in drift kelp transport and delivery to beaches, including hotspots of kelp deposition along the shoreline and strong seasonal patterns of deposition. Fresh kelp plants were most abundant in the lee of headlands and least abundant on narrow stretches of beach with seacliffs or seawalls suggesting the strong influence of coastal topography and beach conditions on drift kelp deposition and retention. Along the 25 km study coast, the number of beached kelp plants varied more than 2 orders of magnitude among months with greatest number of kelp plants (>9000) observed in late summer and the lowest number (<40) in spring. The ending distributions of recovered drift cards and drifter tracks along the shoreline suggest that the majority of drift kelp is deposited on beaches but that proportion varies seasonally. The majority of tagged kelp plants moved <5km before beaching but some tagged kelp plants were reported on beaches much further away, including Monterey Bay and Santa Monica Bay. Loss of tagged kelp plants and kelp plant fate indicated by drifter tracks varied strongly among the 3 study reefs. These observations will allow us to characterize the dynamics of trophic subsidies and the consequences of this variability for recipient beach ecosystems.