FUNCTIONAL COLLAPSE OF A PERSISTENT SHELL-GRAVEL BENTHIC ECOSYSTEM ON THE SOUTHERN CALIFORNIA SHELF WITHIN THE LAST 200 YEARS: PALEOBIOLOGIC DETECTION OF IRREVERSIBLE CHANGE

Death assemblages from the muddy middle mainland Southern California continental shelf (30-90 m) frequently contain shells of epifaunal terebratulid brachiopod and large-bodied scallop species, calcified erect bryozoans, and tube-secreting polychaetes that have not been encountered alive during annual surveys of this area over the last 45 years. Instead, the living community is dominated by infaunal species, especially chemosynthetic and deposit-feeding bivalves. Postmortem age-frequency distributions based on AAR-dating of 190 individuals of the brachiopod Laqueus reveal (1) a mode of ~200 years, (2) no shells younger than 1900 A.D., and (3) a long tail of shells up to 6 ka: for millennia up until the early 1800s, the mainland shelf had apparently been characterized by extensive areas of mud-free, shell-gravel habitat. Extirpation of the shell-gravel community was almost certainly by siltation, given that shell-gravel taxa have been sampled alive only in small patches along the winnowed mainland shelf-slope break and on the still-sandy shelves of the sparsely-inhabited Channel Islands. Faunal decline coincided with 19th Century expansion of grazing and wetland destruction in southern California watersheds; ocean disposal of wastewater, flood control (limiting runoff), and warming are 20th Century phenomena that would have aggravated rather than initiated shell-gravel loss and will frustrate recovery. Shell-gravell loss represents a dramatic loss of habitat heterogeneity at site and landscape scales and an implicit decline of associated finfish and macrobenthic beta diversity. Biomonitoring shows improved seabed health since the 1970s, but the widespread mud on the mainland shelf, a legacy largely of 19th Century, is here to stay, unlikely to be swept clean by natural processes. Faunal responses to 21st Century stressors will occur in the context of this new version of the mainland shelf, permanently ratcheted from conditions that had existed for millennia.