Seasonal cycles, extrinsic factors, and the variable effects of turfing algae on the abundance of a colonial ascidian

Abstract

Factors limiting the local abundance of a subtidal colonial invertebrate Pseudodistoma novaezelandiae (Brewin) (Ascidiacea: Polyclinidae) were investigated at Goat Island, New Zealand. The potential effects on Pseudodistoma abundance of a turfing coralline alga were assessed relative to predation, disease, dislodgement, water-movement, and to the intrinsic, annual cycles of growth of the ascidian. Results from tagging trials and surveys suggested that disease and physical dislodgement had a small effect on Pseudodistoma abundance. Overgrowth affected mainly small colonies that were surrounded by turfing calcareous algae. A strong correlation was found between water-movement (measured by weight loss from erodable hemispheres) and Pseudodistoma abundance. An experiment in which urchins and fish were excluded from Pseudodistoma indicated that the effects of predation were negligible. In a further experiment, trends showed that the manipulative clearance of turfing algae from around colonies, and the infliction of damage to colonies, interacted to depress abundance of Pseudodistoma. This result was contrasted with that from the tagging trials, and it was concluded that the effects of turfing algae on Pseudodistoma abundance depends on the water-movement at the site investigated, and on whether the variable considered is the size of individual colonies or the area covered by the population. Changes in abundance of Pseudodistoma due to the intrinsic cycles of growth were much greater in magnitude than changes due to turfing algae, senescence, predation, biological and physical disturbance, and disease.