Ecosystem impacts of variable recruitment in Antarctic krill investigated with long-term monitoring and archived ADCP backscatter data

Abstract

In this work, I explore the impacts of anthropogenic climate change on the krill-reliant marine ecosystem of the western Antarctic Peninsula. I use long-term ecological monitoring data to examine the impact of highly variable krill recruitment on a krill predator population, and I use archived backscatter data from an Acoustic Doppler Current Profiler (ADCP) to investigate biotic and abiotic drivers of summer krill distribution along the mid-to-coastal shelf region of the western Antarctic Peninsula.In Chapter 1, my coauthors and I examine the impact of cyclical krill recruitment on Adélie penguins. Between 1992 and 2018, the breeding population of Adélie penguins around Anvers Island, Antarctica declined by 98%. In this region, natural climate variability drives five-year cycling in marine phytoplankton productivity, leading to phase-offset five-year cycling in the size of the krill population. We demonstrate that the rate of change of the Adélie breeding population also shows five-year cycling. We link this population response to cyclical krill scarcity, a phenomenon which appears to have arisen from the interaction between climate variability and climate change trends. Modeling suggests that, since at least 1980, natural climate variability has driven cycling in this marine system. However, anthropogenic climate change has shifted conditions so that fewer years in each cycle now prompt strong krill recruitment, triggering intervals of krill scarcity that result in drastic declines in Adélie penguins. Our results imply that climate change can amplify the impacts of natural climate oscillations across trophic levels, driving cycling across species and disrupting food webs. The findings indicate that climate variability plays an integral role in driving ecosystem dynamics under climate change. In Chapter 2, I explore the viability of using archived Acoustic Doppler Current Profiler (ADCP) backscatter data to examine krill distribution on the WAP. During the Palmer LTER’s oceanographic cruises, the ship runs an ADCP the entire time it is at sea. An ADCP is a sonar instrument designed to measure water velocity across the water column, but the collected data can also be repurposed to provide information on the distribution and density of sound-scattering objects. Therefore, in regions where the primary sound-scattering objects are zooplankton, the ADCP can be used to map zooplankton distribution. I explore whether archived data (2005-2018) from an ADCP on Palmer LTER sampling cruises can be retroactively analyzed to provide information about the amount and distribution of krill in the water column along the western Antarctic Peninsula. I found that, given the uncertainty on several key instrument parameters, the ADCP’s estimates of krill biovolume are likely to have uncertainty spanning at least an order of magnitude and therefore the ADCP cannot on its own be used to estimate absolute biovolume. However, available evidence suggested that variability in seawater properties and ADCP system parameters is either low and/or can be accounted for using available measurements, and therefore meaningful relative biovolume estimates can theoretically be achieved. Several challenges – including the mismatch between depths sampled by the ADCP and those sampled by net tows, as well as the fact that krill are not the only species present in the sampled region – add uncertainty to comparisons between ADCP data and ground-truthing data available from tows. However, I found highly significant though noisy empirical relationships between the biovolume of krill in tows and the backscatter coefficient calculated from the ADCP, indicating that the ADCP does give useful information about the amount of krill and can be used to map krill distribution along the ship track. In Chapter 3, I use the ADCP backscatter data from Palmer LTER cruises to explore the effect of abiotic and biotic factors on krill distribution along the mid-to-coastal shelf region of the WAP. I find that krill recruitment around Palmer Station is more closely linked to krill density 600km south than to krill density on the sampling line next to Palmer Station, suggesting that krill density further south along the Peninsula is more dependent on recent recruitment than krill density further north. Net tows support this idea, indicating that a larger proportion of recruits are found in the south of the grid than in the north. I also determine that the distance between patches of high biomass becomes exponentially greater with increasing time since a high krill recruitment event, indicating that the foraging conditions become much more difficult as time elapses since a high krill recruitment event. I find evidence that krill are spatially correlated with areas of high primary productivity, suggesting that krill may move to aggregate in these areas and that changing distributions of primary productivity under climate change are likely to change krill distributions. I examine whether krill show avoidance behaviors, such as retreating to colder water at depth, in response to high summer temperatures, but I find no evidence of this and even find a spatial correlation between krill and areas of higher temperature. I conclude that krill are likely not yet temperature-stressed in this region of the WAP. Overall, lowered primary productivity and krill recruitment – and perhaps temperature as it becomes even warmer – will likely have major impacts on the distribution of krill, and therefore on their accessibility to predators and fisheries.