The trophic implications of morphological plasticity in Northeastern New Zealand Ecklonia radiata forests

Abstract

Morphology is an overwhelmingly influential component of an organism’s ecology, not only determining how an organism interacts with its environment, but how it interacts with adjacent communities. Kelp forests are thought to play an important role fuelling reef secondary production through the utilization of detritus. It is assumed that a proportion of kelp detritus is within the size fraction of particulates ingested by suspension feeders, and that kelp particulate makes up an important component of particulate organic matter on temperate reefs. However, the production of kelp detritus and therefore its importance for suspension feeders may differ between kelps of differential morphological form. This thesis proposed that the production of kelp particulate through tissue erosion would vary between morphologically distinct populations of Ecklonia radiata across north-eastern New Zealand, thus varying the trophic influence of E. radiata among kelp forests of differing morphologies. This thesis quantified morphological variation in E. radiata across wave exposure and depth gradients throughout north-eastern New Zealand, as well as providing measures of productivity and tissue erosion across these gradients using both an established and novel method, respectively. Furthermore, this study determined the level of kelp detritus utilization in benthic suspension feeders via stable isotope analysis employing both a laboratory and field approach, involving an orthogonal experimental design controlling the presence of kelp, and depth-stratified field collection across sites of varying wave exposure. E. radiata morphology was found to show a non-linear relationship with wave exposure as shown by upper quantile spline regression analysis. Surprisingly, individuals at both the most exposed and most sheltered locations were found to share similar morphologies with short stipes and long primary lamina. However, the short stipe morphology is likely derived from contrasting mechanisms of maximising light capture and mass transfer of nutrients at sheltered locations and minimising drag at very exposed locations. In contrast, individuals inhabiting moderately exposed locations exhibited comparably long stipes and short primary lamina. This morphology is considered unique to North-eastern New Zealand, when compared to E. radiata from temperate Australia and Fiordland, which are characterised by thalli proportionally dominated by lamina. Overlying this variation in morphology, individuals showed significant morphological variation with season, due to increased lateral production over spring. Despite high variance in morphology, rates of productivity and erosion were constant across morphologically distinct populations indicating that detrital production does not vary in a consistent manner with morphology. Experimental addition of kelp detritus in tanks did not enhance growth of a variety of common suspension feeders and isotope analysis did not provide evidence that the suspension feeder’s diet were subsidized by kelp carbon. Based on field collections the contribution of E. radiata was shown to be dependent on species, site, and depth. E. radiata was found to be a high dietary contributor for the sponge Ancorina alata and the solitary ascidian Cnemidocarpa sp., but only at 14m depth at Goat Island. The overall contribution of E. radiata to suspension feeder diets and also to the POM was generally found to be low. In general the stable isotope analysis provides support for the argument that inferring trophic connections between macro-algae and benthic suspension feeders via stable isotope analysis and the employment of mixing models can be ambiguous. Overall, this thesis concludes that despite high morphological plasticity in E. radiata forests across north-eastern New Zealand, this variation has little influence on E. radiata’s trophic importance to benthic suspension feeders. Furthermore, the multiple approaches used in this study suggest that kelp detritus is of less trophic importance to benthic reef suspension feeder communities than previously thought.