Impact of seasonality on a key benthic grazer and its role in ecosystem function

Abstract

Seasonal variation impacts all aspects of ecosystems including temperature, nutrient availability, and photoperiod, influencing everything from individual physiology to community-wide food availability. Hong Kong experiences large seasonal variations in environmental conditions, especially seawater temperature and algal biomass, but little is known about how this seasonality affects benthic herbivores. Therefore, I used a suite of approaches, from lab-based temperature experiments, physiological measures, and fatty acid analysis, to long-term in situ experiments, to investigate the effects of seasonality on a key subtidal herbivore in Hong Kong, and how its role within the ecosystem changes with fluctuating environmental conditions. To determine which winter-abundant algae the sea urchin Diadema setosum feed on, I surveyed urchin densities at macroalgal communities while identifying the five most abundant algae. I used feeding assays to determine preference, grazing rate, and the effects algal diet on urchin growth. Urchins preferred Sargassum hemiphyllum when presented with all five algae species simultaneously, which coincided with increased growth rates. When only given a single algal species, however, urchins grazed more rapidly on algae with lower nitrogen content, possibly to meet nutritional needs. I then tested if urchins would increase their energy stores in winter for the food-limited, energetically demanding summer. Contrary to predictions, triacylglycerol concentrations and total energy stores of urchins did not differ seasonally. Yet, metabolic rates quadrupled in summer while grazing rates only doubled, suggesting lower winter metabolic rates may limit feeding activity. I calculated seasonal metabolic energy demands, comparing them to energy consumed and found that urchins could meet their energy needs by consuming rapidly growing algal turfs which are twice as calorie-dense as macroalgae. I hypothesized that D. setosum would alter their fatty acid profiles in winter to increase membrane fluidity to maintain function yet may also increase susceptibility to oxidative stress. Surprisingly, I found that urchins decrease the amount of double bonds which decreases lipid fluidity, possibly as a response to increased lipid peroxidation in winter. However, urchins in temperature-controlled laboratory experiments did not have increased lipid peroxidation meaning that oxidative stress is caused by more than cold temperatures. Grazers maintain coral health by supressing algal growth, but this role is largely unknown in eutrophic systems (e.g., Hong Kong). To test this, I experimentally excluded urchins for 13 months to assess grazing effects on survival and growth of three coral species. I quantified algal settlement and sediment accumulation, bioerosion, coral growth and survival. Urchins greatly reduced algal settlement and sediment accumulation; however, coral growth and survival was species-specific, with one species increasing in survival and growth, a second species positively impacted for survival but unimpacted for growth, and a third species negatively impacted for both when urchins were present. Urchins were particularly beneficial to two of the three species in winter, when corals were potentially cold stressed and algal growth was high. In this thesis, I elucidate seasonal effects on an important benthic grazer and their function within the ecosystem. As the effects of seasonal variation increase, this can increase stress on urchin physiology and influence their functionality; therefore, understanding seasonal variation is invaluable to understanding current and future stressors on ecosystem engineers such as Diadema setosum.