The bottom mixed layer depth as an indicator of subsurface Chlorophyll a distribution

As we begin to manage our oceans and shelf seas for more complex simultaneous uses, such as renewable energy development, fishing, and marine protected areas, it is becoming increasingly important to understand the details of primary productivity at fine spatial scales. Besides very shallow waters, the vast majority of phytoplankton production in continental shelf waters generally occurs under stratified conditions, where the pycnocline provides a stable habitat for phytoplankton growth in the lower euphotic zone. The seasonal heating–cooling cycle of the water column regulates the stratification in temperate shelf waters, where the intensified solar radiation in spring–summer increases the difference of temperature and salinity between surface and deep waters and prompts the formation of a pycnocline dividing the surface from deep mixed waters. Once the stratification is set in spring–summer, turbulent mixing represents the main source of new nutrients into the pycnocline during prolonged stratified conditions. Climate change (Holt et al., 2016, 2018) and the introduction of numerous man-made infrastructure (e.g. offshore wind farms, Dorrell et al., 2022) are expected to alter the balance between mixing and stratification in shelf regions, affecting the vertical exchange of nutrients between deep and surface waters (below and above the pycnocline). Anomalies such as circulation slowdown, sea-level rise, bottom and surface temperature, wind speed, and wave height have largely been described as a consequence of climate change in the last 2 decades (e.g. Orihuela-Pinto et al., 2022; Taboada and Anadón, 2012; Bonaduce et al., 2019), while the consequences of these physical changes on the biological processes are still only partially understood (Lozier et al., 2011; Somavilla et al., 2017).