Diet of the West Coast rock lobster Jasus lalandii: influence of lobster size,sex, capture depth,latitude and moult stage

Diets of male and female West Coast rock lobster Jasus lalandii in South Africa were compared across a large size range of 10–85 mm carapace length (CL). The diets of male rock lobsters were compared between two different depths, different seasons, across the moult cycle, and among eight sites along the South-Western Cape coast. There was no significant difference in diet between male and female rock lobsters for any of the size-classes examined. Male rock lobsters showed large differences in diet between small and large size-classes. The diet of small lobsters (<75mm CL) consisted of a wide range of species, which included, in order of importance, coralline algae, barnacles Notomegabalanus algicola, sponges and ribbed mussels Aulacomya ater. However, prey items rich in inorganic material were not dominant in their diet, as had been predicted. By contrast, large rock lobsters (>80mm CL) fed on few species, and fish and ribbed mussels were their most abundant prey items. There were some dietary differences between individuals captured at 20 m and those collected at 50 m, but these differences were less marked than between the two sampling sites (the Knol and Olifantsbos). There was seasonal variation in diet at the Dassen Island and Olifantsbos sites. Cannibalism was highest during the moulting periods. Gut fullness varied seasonally at Dassen Island, and was consistently high at Olifantsbos. However, the proportion of the population feeding showed marked seasonal trends at both sites, tracking the commercial catch per unit effort of rock lobster. Ribbed mussels were a ubiquitous and dominant component of the diet at the eight sites sampled. However, south of Dassen Island, black mussels Choromytilus meridionalis were scarce in the diet of rock lobster and sponges predominated. Gut fullness was lowest at the northernmost sites.     

The effect of water transport on nitrogen flow through a kelp-bed community

A simulation model is used to investigate possible ecological effects of up- and downwelling on nitrogen flows through a kelp-bed food web off the Cape Peninsula, South Africa. The model depicts the flow of nitrogen, which is often limiting in marine ecosystems, from kelps, other macrophytes and phytoplankton, through filter-feeders to carnivores, with a feedback loop via faeces and bacteria to detritus and filter-feeders. When modelled as a closed system, bacteria associated with detritus and animal faeces form a large component of the particulate nitrogen available to filter-feeders, and the faeces feedback loop dominates nitrogen flow. When measured rates of water transport are incorporated into the model, bacteria have little opportunity to accumulate before being transported out of the system. Animal faeces and kelp detritus are the dominant filter-feeder food components under upwelling conditions, whereas phytoplankton is the major contributor to particulate organic nitrogen under downwelling conditions. When realistic pulses of upwelling/downwelling derived from wind indices are used as model input, filter-feeders are shown to decline during the summer upwelling season when much potential food is advected out of the system, and they increase during the winter when downwelling conditions are more prevalent, bringing in nitrogen-rich phytoplankton from the blooms developing offshore.