Meteorological controls on primary production in shelf seas

Abstract

Shelf seas are regions of high biological activity, contributing 15-30% of global oceanic primary production, with temperate shelf seas as an important global carbon sink. To understand how shelf seas will respond to environmental changes it is important to fully understand phytoplankton dynamics and inter-annual variability of phytoplankton production in these areas. Previous modelling works have shown that meteorology can affect phytoplankton seasonal dynamics but there is still debate in the literature about the direct mechanisms that affect long-term phytoplankton productivity. Challenges can include a lack of long-term observations and mismatch of these with numerical models due to a range of factors from the simplicity of the assumptions made in model structure to inappropriate parameterisations. This study explores the effects of wind speed, cloud coverage, air temperature, and relative humidity on primary production using a simple one-dimensional biological and physical coupled model to simulate the inter-annual variability of seasonal stratification and productivity over the last five decades (1965 - 2015) and shelf sea observations (2014 - 2015) in the Central Celtic Sea location. Results with the simplest model structure show that wind speed has the largest effect on the inter-annual variability of primary production associated to the onset of thermal stratification. However, a mismatch with observations and inconclusive results motivated further model development, introducing a nutrient-phytoplankton-zooplankton framework and modelling photo-acclimation. Extensive analysis of the new models was performed by calibration and sensitivity analyses, demonstrating that the addition of these processes produces substantial changes in the model dynamics, improving the model fit to observations. This allows reassessment of the questions analysed with the simplest model, providing new insights into the meteorological impacts on phytoplankton productivity, showing that wind speed has a direct influence on the timing of thermal stratification and, therefore, in the timing of the spring bloom, affecting annual production. In contrast to the simpler model, cloud coverage is shown to have the largest effect on the annual phytoplankton production, affecting the available light in the water column with large daily variations directly affecting daily primary production during the spring bloom and summer growth periods. Thus, in this work it is demonstrated that the structure and parameterisation of the model influences the fidelity of the simulations.

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ORCID for Anna Hickman: orcid.org/0000-0002-2774-3934

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