Physical, biological, and chemical inter-processes in fjords.

Abstract

Physical, chemical, and biological processes in fjord ecosystems are inherently intertwined and their interactions cultivate environments ideal for primary production and ecological diversity. Yet, due to the vast range of space and time scales important in fjords, defining specific inter-process outcomes, such as conditions that trigger harmful algal blooms (HABs), has proven to be a challenging task. To identify repeatable inter-process patterns this study will focus on time scales over which plankton are known to depend on and vary, such as the change from day to night, or the diurnal cycle. Therefore, in December 2021, a field experiment was conducted in a fjord of northern Chilean Patagonia (41.6º S) to capture the interaction of physical, chemical, and biological processes for a complete diurnal cycle (24 h). The measurements collected included hydrographic, atmospheric, tidal, and nutrient data as well as phytoplankton and zooplankton samples to elucidate how specifically these processes interact. Results detected thin, near-surface phytoplankton and zooplankton layers during the daytime, located just below a layer of fresh water, acting as a barrier. During the night the thin layers were dissipated and dispersed. Predator-prey interactions were one of the factors contributing to phytoplankton dissipation at nighttime, due to the diel vertical migration of macrozooplankton species. The physical measurements showed reduced stratification and enhanced vertical mixing homogenizing the upper layers of the water column at night, which is thought to be enhanced by the presence of swimming macrozooplankton. Diatoms dominated the phytoplankton composition, but HABs species were observed, showing changes in abundance and species composition from day to nighttime. This study reinforces the need to carry out interdisciplinary experiments to understand how physical, biological, and chemical processes in fjords interact, to forecast and mitigate the effects of water quality issues such as harmful algal blooms.