Contrasting impact of diatoms on dominant calanoid and cyclopoid copepods in a temperate bay

Abstract

The role of diatom dominance and phytoplankton concentration in determining the abundance of the dominant calanoid and cyclopoid copepod species in Port Phillip Bay, southern Australia, was investigated. Sampling for Chlorophyll-a, phytoplankton and zooplankton was conducted on 11 sampling trips between August 2016 and June 2017. The proportion of diatom cells was positively related to the total phytoplankton cell concentration but less related to the chlorophyll-a concentration. Abundance of the dominant calanoid copepod, a key prey item for larval fish, increased with water temperature but declined when the proportion of diatom cells increased, and was unrelated to chlorophyll-a concentration. In contrast, abundance of the dominant cyclopoid copepod was not related to the proportion of diatom cells but was positively related to water temperature and chlorophyll-a concentration. The results verify the assumptions previously made about the negative relationship between the proportion of diatoms and abundance of the dominant calanoid copepod Paracalanus indicus when modelling planktonic production and fish recruitment in Port Phillip Bay. In relation to the dominant cyclopoid copepod Oithona similis, the results indicate that this species was not affected by the proportion of diatoms. Instead, the results suggest that food-limitation may be a key factor determining the abundance of O. similis in Port Phillip Bay. Overall, variation in zooplankton composition and fish recruitment in Port Phillip Bay will depend not only on phytoplankton abundance but also on the composition in relation to the dominance of diatoms.