Copepods are a dominant component of mesozooplankton communities and are unambiguously important in marine food webs. Climate change and rising sea surface temperatures are likely to induce biogeographical range shifts and phenological changes for all zooplankton species, which include copepods. Consequently, consistent time-series are fundamental to monitor the status of plankton communities and predict future changes that can greatly affect the whole food-chain. Here, a recent time series (2014–2020) of image-based copepod abundance data from the Belgian Part of the North Sea has been explored and compared with previously collected (2009–2010) microscopy-based datasets for the same study area. The time series revealed distinct discrepancies in copepod phenology, abundances and total biomass between nearshore and offshore regions. The nearshore area was characterised by an altered copepod dynamics through time, with the occurrence of a single annual spring peak in both abundance and total biomass, and no autumn peak usually described and present in that area. Conversely, the autumn peak was clearly observed at the offshore stations, together with a distinct additional early spring peak in comparison to the nearshore and midshore regions which has not been documented before. Remarkable anomalies were detected within the time-serie of copepod for both abundance and biomass, notably in autumn of 2018 and in a lesser extend in 2014 and 2019, with values dropping to near-zero for the nearshore stations. The various heat waves during the summer periods of these years are considered potential causes for these copepod decreases, since they corresponded to the physiological thermal limit of many copepods species known to be present in the area. These heat waves events could have also favoured predators of copepods such as jellyfishes as well as enhancing the occurrence of d harmful algal blooms. Both anomalies in 2018 and 2019 actually coincided with important phytoplankton autumn blooms. The results from this copepod time-series analysis illustrates the changes affecting this essential marine trophic level and highlights the value and relevance of copepod long-term data series in the context of climate change and ecosystem management. |
