title: Universal ecological responses to climate change: understanding impacts on zooplankton
Creators: name: Corona, Stefano
Publisher: name: University of Liverpool Repository
Year: 2023

description: Copepods, typically dominate marine metazoan zooplankton both by numbers and biomass. They are a fundamental energetic link between the basal and the upper layers of the marine food web, and contribute substantially to the biogeochemical cycling of carbon. Copepods, being ectothermic and short-lived animals, are sensitive to temperature and thus represent good indicators of global warming effects. I use this group to investigate the three so-called “universal responses to climate change”: the temperature-size response (TSR), by which adult copepods present a reduced body size when reared at warmer thermal regimes; the phenological shift, which is investigated indirectly in copepod populations using the change in timing of seasonal abundance through the year; and the biogeographical shift, which is the change of latitudinal distribution of abundance to keep pace with the poleward shift in isotherms. I aim to investigate and compare each of these three responses in detail across seven ecologically important copepod species. First, I explore the body size change response by measuring the body size of copepods collected through the English Channel station L4 time-series (32 years). Besides calculating the percentage change of body mass per degree Celsius across multiple years and months, I also detected signs of long-term declines in body size with warming. On top of this, I further revealed a correlation between body size and copepod abundance at the beginning of their temperature-dependent phase (in April), and the correlations with the high zooplankton/phytoplankton ratio at this time of year suggested that body size reflected a trade-off with predator avoidance as well as food acquisition. Secondly, I used three multi-decal times-series of plankton abundance across the North-West European shelf (at L4, Stonehaven, and Helgoland Roads), to estimate the species-specific phenological and thermal niche change with temperature. Shifts in timing of seasonal abundance across years were inconsistent among species and stations and were explained more by time than temperature. Nevertheless, I detected signs of thermal niche conservation manifested through shifts in seasonal timing of abundance across different latitudes and across years. However, I suggest that the real phenological adjustment in some stations may have been obscured by the long-term decline in abundance at specific times of the year. Third, I investigated the thermal niche conservation that copepod populations demonstrate through biogeographical adjustments, by using the extensive Continuous Plankton Recorder (CPR) survey dataset. Latitudinal shifts with warming were detected, but these were stronger and more consistent in the North-East Atlantic (where species conserved their thermal niche over time) than in the North Sea. In the end, I combined the above results to explore how the “universal” responses to climate warming were linked. I found that species that can adjust phenotypically by shifting in adult body size with warming tended to show the least degree of phenological shift to counteract warming. The intensity of shifts in seasonal timing and in latitude across species appear to be positively correlated: thus, species that shift most toward spring also shift northward more intensely. Moreover, in the North Sea, the phenological shift toward spring was found to be stronger at northern latitudes than at southern ones. Consequently, these three responses to warming need to be considered together in future studies that aim to better understand and possibly predict the long-term change in zooplankton spatio-temporal structure.