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Modeling of intra-annual abundance distributions: constancy and variation in the phenology of marine phytoplankton species over five decades at Helgoland Roads (North Sea)

Scharfe, Mirco, Wiltshire, Karen Helen
Ecological modelling 2019
biomass, climate change, environmental impact, marine ecosystems, marine environment, models, phenology, phytoplankton, spring, summer, water temperature, North Sea
Annually recurring environmental processes such as the cycle of temperature and light drive the phenology of marine plankton populations. Improved knowledge about the homogeneity and amplitude of the phenological response of phytoplankton to climate change is essential for an assessment of ecological consequences on the marine ecosystem. We analyzed phenological variability of 21 phytoplankton species work-daily monitored at Helgoland Roads from 1962-2015. We used a function of Weibull-type to estimate phenological dates of species-specific abundance peaks. The combination of derived dates and associated peak abundances formed the basis for the analysis of long-term changes in phenological time slots and associated environmental conditions.Species-specific preferences in combination with seasonally varying environmental trends resulted in a complex pattern of phenological long-term response. Phenological trends reflect both constant occurrence and shifts to an earlier or later occurrence. Even co-occurring phytoplankton species may exhibit different phenological trends within identical time slots. Differences in species-specific trends in timing also reflected the seasonally varying shifts in water temperature ranges due to warming. In spring and summer, the main patterns of common variability in timing were associated with different abiotic and biotic drivers. The majority of species showed more narrow time slots with regard to the occurrence of higher peaks. Considering the variation of species occurrence in such a “typical” time window provided further insight in terms of assigning the effect of environmental drivers on inter-annual phenological variation. Phytoplankton species with similar long-term trends in timing (in days) showed different trends in biomass, i.e. the phenological changes resulted from different ecological responses to environmental change. The local character of environmental trends underlined the limitations of the applicability of findings between to different measuring sites or wider areas such as the North Sea. The study emphasizes the benefit of a highly resolved phytoplankton record for understanding long-term ecological changes in a highly dynamic marine environment such as the North Sea.