Seasonal variation of zooplankton nutritional quality at a reef!Manta Ray (Mobula alfredi) Feeding ground on Ningaloo Reef

Alexandra Thornton

Zooplankton provide a fundamental connection between primary producers and higher trophic level consumers, supporting some of the largest marine animals as well as microbial organisms. Therefore, the nutritional resource of zooplankton must be sufficient to support a wide array of marine species. The nutritional quality (lipids, protein and carbohydrates) varies seasonally with changes in species composition, as different organisms store varying amounts of biochemical components. These seasonal variations have direct effects on the marine food web, and those organisms that rely upon the nutritional resource. Zooplankton has a spatiotemporally patchy distribution, with variations in species composition. This is reflected by the variability of each biochemical component when comparing regions around the world. This study aimed to quantify the seasonal proportions of zooplankton nutritional components at Bateman Bay, Ningaloo Reef in Western Australia, in order to determine what planktivorous fish such as reef manta rays (Mobula'alfredi) are receiving year round. Feeding manta rays were used to locate patches of plankton, and samples were collected via towing a 300 μm mesh plankton net from the back of a boat. Samples were collected from four sites in autumn and three sites in winter. All samples were analysed for lipids, protein, carbohydrates, total organic carbon and nitrogen (C:N ratio) and biomass. All nutritional components varied significantly between seasons, and each site showed compositional variability, especially protein. Protein and lipids were significantly higher during autumn than winter, while carbohydrates were higher during winter. The C:N ratio was significantly higher during winter, when phytoplankton abundance was higher. Biomass was larger in winter when there was a greater abundance of portunid crab larvae, eggs and phytoplankton, however was not statistically significant. Environmental variables; temperature, turbidity and tide had no correlation to zooplankton biomass, nutritional value and reef manta ray feeding. However, greater nutritional quantities were found when manta rays were feeding, and the highest biomass was recorded when the largest feeding aggregation of reef manta rays was observed. This study has provided insight into the biochemical composition of mixed zooplankton populations around Bateman Bay, Ningaloo Reef in autumn and winter. It appears that manta rays match their distribution to the zooplankton, and nutritional quantities proved to be higher during warmer temperatures. However, additional year-round sampling is recommended for future studies, in order to better understand these preliminary findings. Knowledge of how manta rays rely on areas with high nutritional value will help to explain seasonal variations in reef manta ray visitation, and provide management implications for the conservation of this species.

Seasonal variation of zooplankton nutritional quality at a reef!Manta Ray (Mobula alfredi) Feeding ground on Ningaloo Reef

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