Evaluation of excess [sup 234]Th activity in sediments as an indicator of food quality for deep-sea deposit feeders.

Deep-sea deposit feeders selectively ingest large volumes of sediment. Knowledge of the nature of this selectivity will help to elucidate the limiting nutritional requirements and geochemical impacts of these abundant animals. Shallow-water and theoretical studies suggest that deep-sea deposit feeders should select particles rich in protein, bacterial biomass, and/or chloropigment concentrations. Recent studies indicate that deep-sea megafaunal deposit feeders exhibit strong gut enrichment of excess (xs) [sup 234]Th activity, even though [sup 234]Th[sub xs] lacks nutritional value. To explore the significance of selective ingestion of [sup 234]Th[sub xs] activity, we evaluated the correlations between [sup 234]Th[sub xs] activity and three potential tracers of deposit feeder food quality: chlorophyll a (chl a), enzymatically hydrolyzable amino acids (EHAA), and adenosine triphosphate (ATP). Surface sediments from three quiescent bathyal basins off Southern California (San Nicolas, Santa Catalina, and San Clemente) were collected by a multiple corer and analyzed for [sup 234]Th[sub xs] activity, chl a, EHAA, ATP, and total organic carbon and nitrogen. [sup 234]Th[sub xs] activity was positively correlated with chl a and phaeopigment concentrations and negatively correlated with EHAA concentrations. Excess [sup 234]Th was not linearly correlated with concentrations of ATP, organic carbon, or total nitrogen. The results suggest that deep-sea deposit feeders select sediments with high [sup 234]Th[sub xs] activity because it is associated with recently settled phytodetrital material. There is no evidence that this [sup 234]Th[sub xs]-rich material has particularly high concentrations of labile amino acids or microbial biomass. Phytodetrital material may be an important source of some other limiting nutrient to deep-sea deposit feeders, e.g., polyunsaturated fatty acids, labile organic carbon and/or vitamins. [ABSTRACT FROM AUTHOR]