Copepod gut contents, ingestion rates, and feeding impacts in relation to their size structure in the southeastern Taiwan Strait

Abstract

Copepod gut contents, ingestion rates, and grazing impact on phytoplankton were estimated at 19 sampling stations off the Penghu Archipelago (the Pescadores), in the Taiwan Strait during the summer monsoon season. Copepods were collected on board the Ocean Research Vessel III of the National Science Council within the upper 5 m by horizontal tows using a NORPAC 4.5-m-long conical net with a 1 m opening diameter and a 333 μm mesh size. The gut fluorescence method was used to estimate in situ ingestion rates and clearance rates of copepods. The abundance and species composition of copepods varied among sampling stations. A size-fractionation approach was applied, and the gut pigment contents and ingestion rates were estimated in 3 size categories (small, ＜1 mm; medium, 1-2 mm; and large, ＞2 mm). Results showed that gut pigment contents were higher for larger copepods and lower for smaller ones. Estimates of the copepod grazing impact showed that ＜ 1% of the total phytoplankton standing stock was grazed daily during the early summer monsoon period. Grazing impact for the 23 most abundant taxa, which accounted for up to 90% of all copepods counted at each station, demonstrated large spatial variability ranging 0.001%-0.210% of the chlorophyll a standing stock. The highest grazing impact was recorded at station 18, a coastal station situated in the Penghu Channel, southwest of Penghu I., whereas the lowest impact was recorded at station 11 near the center of the Strait, northeast of Penghu I. The smaller copepod fractions were responsible for most of the grazing impact, due to their numerical dominance in these coastal waters. Variables affecting copepod community grazing impact estimates are discussed.