Nutrient Dynamics of Coastal Phytoplankton: The Role of Episodic Flow Events and Chronic Sewage Discharges

Abstract

Nutrient enrichment of coastal zones can lead to longer phytoplankton bloom seasons, shifts in phytoplankton community composition and a higher frequency of harmful algal bloom events (HABs). Predicting the effect of nutrient enrichment on any given coastal system is challenging, as the response is mediated by the physical and biological characteristics of the system and the way in which nutrients are delivered to the system. For example, in tropical and sub-tropical regions, the majority of annual nutrient loads are delivered via episodic high-flow events during the wet season. These events stimulate phytoplankton growth and are known to represent an important driver of coastal productivity. However, in populated coastal settings other nutrient inputs from point sources, such as sewage treatment plants (STP), are typically delivered as a relatively constant discharge. While chronic STP nutrient loading may be relatively low compared to flow event inputs, their continuous delivery throughout the year may represent a significant source of nutrients for phytoplankton growth, particularly during the dry seasons. These different sources of nutrients, and the spatial and temporal characteristics of their delivery to receiving waters, likely influence the responses of phytoplankton communities.