Grazing impact of salp (Tunicata, Thaliacea) assemblages in the eastern tropical North Pacific

Abstract

Salps are gelatinous filter-feeders that can intermittently contribute significantly to the cycling of organic matter in the world’s oceans. We estimated the grazing impact of salp assemblages on phytoplankton biomass and rates of primary production (PP) in the eastern tropical North Pacific off Mexico during two cruises in February and November of 2003. Salp biomass comprised a large proportion of the preserved zooplankton biomass (21‐525 mL per 10 3 m 3 ) at stations where large salps were present. Of the 19 species found in the area, Thalia democratica (and closely related species) comprised 11‐100% of total salp abundance but were surpassed in dominance at some locations by Metcalfina hexagona (49%) and an unidentified Cyclosalpa (29%). In February, salp abundance ranged from 6 to 1901 salps m 22 (0.1‐13.5 mg C m 22 ); in November, values ranged from 54 to 631 salps m 22 (1‐193 mg C m 22 ). Ingestion rates by the salp assemblage, estimated from gut pigment content, were moderate and highly variable. Thalia exerted a low impact on phytoplankton stocks and PP, whereas moderate aggregations of cooccurring M. hexagona, Cyclosalpa sp. and Pegea confoederata exerted a higher impact when present. Salp assemblages ingested 0.01‐3.5% of chlorophyll standing stock daily. The proportion of PP removed by salps was estimated to range from 0.1 to 24.5% day 21 , suggesting a significant role for salps in the grazing mediated