An integrated study of a rapidly changing continental shelf ecosystem : linking physical conditions, prey field dynamics and top predator behaviour through a marine heatwave event

The land-sea interface provides some of the world's most valuable and biodiverse habitats, despite being exposed to anthropogenic pressures. Marine predators which cross this interface are particularly vulnerable, with human activities in coastal zones diminishing both the quality and availability of suitable breeding and foraging areas. These predators are constrained to forage in smaller oceanic regions while rearing young on-land, therefore their reproductive success is intrinsically linked to the productivity of nearshore waters. Changing environmental conditions, as a direct and indirect consequence of climate change, can alter the structure, distribution and community composition of lower-trophic level prey. In order to predict the response of ecosystems to this change, and to changes as a result of extreme events such as marine heatwaves (MHW), an in depth understanding of the links between environmental factors, prey-field dynamics and predator behaviour is needed. The continental shelf to the south-east of Tasmania is a hotspot of biodiversity, where seasonal productivity supports a large and diverse array of marine birds and mammals. However, this region is also subject to rapid environmental change, being situated within the south-east Australian climatic-hotspot. Due to the intensification and increasing southward penetration of the East Australian Current (EAC), a major western boundary current running from the sub-tropical Coral Sea to the south-east coast of Australia, warming is occurring at an accelerated rate. Quantifying how prey-field dynamics respond to these changing environmental conditions, and the flow-on effects to the behaviour of apex predators, formed the main objective of this study. Surveys were conducted over a three-year period (2015-2018), during which a prolonged marine heatwave (MHW) event occurred that increased water temperatures of the entire western Tasman Sea by a mean of 2.9°C above climatology for 251 days. To develop an integrated understanding of ecosystem dynamics through a period of high environmental variability, zooplankton prey-field dynamics, fish school presence, little penguin (Eudyptula minor) foraging behaviour, and the distribution and abundance of key bird species were analysed in relation to local environmental factors. Zooplankton community composition and abundance were examined in relation to environmental drivers. Generalised additive models (GAMs) indicated a significant decrease in community abundance during the MHW, with a shift in species assemblages away from temperate species and towards EAC-associated species. The size structure of the zooplankton community was also analysed using the normalised biomass size spectra (NBSS). The NBSS is an effective way to demonstrate the variability present in a community, in terms of gains and loss of energy through respiration, predation and mortality. It can also be indicative of changes to the equilibrium of a community. Strong seasonality was detected in the results, with temperature, current velocity and mixed-layer depth being significant drivers of variability in the NBSS. These lower trophic level dynamics were linked to the behaviour of top predators through a detailed case study of the at-sea habitat preference of little penguins breeding in south-east Tasmania (n=13). Tracking was conducted over two summer periods, in 2016 during the MHW, and in 2018 under cooler and more stable environmental conditions. Habitat models (species distribution models) were developed to asses spatial distribution patterns and examine the bio-physical factors influencing foraging trips at fine-scale. Regions of higher sea-surface temperature gradients and cooler than average temperatures were found to increase the probability of penguin presence. The predictability of little penguin habitat-use according to prey-type was also assessed by including covariates for the general distribution of resources in the region; e.g. total zooplankton abundance, and the abundance of Australian krill (Nyctiphanes australis), which forms part of little penguin diet. Little penguin foraging areas were more influenced by the distribution of Australian krill than by general zooplankton abundance. The response of local predators to changes in bio-physical parameters were measured by modelling the distribution of 10 species of seabirds using boosted regression trees. Key species ranged from small planktivores, such as the common diving petrel (Pelecanoides urinatrix), to albatross (family Diomedeidae). Therefore, to encompass the range of prey that underpins the distribution of these species, biological covariates included zooplankton biomass, and the presence (and absence) of fish schools (determined using hydro-acoustics during surveys). Seabird species were separated into feeding groups using multivariate analysis and modelled separately to reveal potential drivers for each group. Despite different biological predictors influencing the distribution of different groups, sea surface temperature was found to explain the greatest amount of variation across all feeding groups. This influence is thought to be prey-mediated, as both biological covariates tested exhibited negative correlations with increasing SST. Through considering the complex links which exist between predators, their prey and the physical environment, this study produces new insights into the potential effects of extreme events. Further, it improves our understanding of how general warming trends affect prey structure and the possible flow-on effects for predators. Modelling the distribution of apex predators enables the identification of important foraging regions with favourable bio-physical characteristics. We highlight how detailed assessments of ecosystem and environmental interactions can be pivotal to informing the effective management of these vulnerable and biodiverse ecosystems into the future.