Prey Size Spectra and Predator to Prey Size Ratios of Southern Ocean Salps

Salp grazing is important in shaping planktonic food-web structure. However, little is known about the size ranges of their prey in the field or how grazing impacts size structure. This study investigated the feeding habits of both the solitary and aggregate life stages of 7 different species of salps, representing a variety of sizes across subtropical and subantarctic waters east of New Zealand. Scanning electron microscopy was used to examine the gut contents of 58 salps, which were then compared to water column plankton communities characterized via epifluorescence microscopy, FlowCam, and flow cytometry. While most of the gut contents resembled ambient waters, substantial differences were found amongst some co-occurring species, such as increased retention of submicron bacteria by Thalia democratica. We found that even for those salps capable of feeding on bacteria efficiently, nanoplankton and small microplankton still made up the majority of gut biomass. Larger microplankton were rarer in the guts than in the water column, suggesting an upper size-threshold in addition to the lower size-threshold that has been the focus of most previous work. Salp carbon-weighted predator to prey size ratios were variable, with the majority falling between 1,000:1 and 10,000:1 depending largely on the size of the salp. Taken together our results indicate that despite being able to feed on submicron particles, picoplankton make up at most 26.4% (mean = 6.4%) of salp gut carbon and are relatively unimportant to the energetics of most salps in this region compared to nanoplankton such as small dinoflagellates and diatoms.