Development of harmful algal blooms in a coastal lagoon: the influence of physicochemical processes and phytoplankton ecophysiology

This study was conducted in Esquimalt lagoon, located southwest of Victoria, British Columbia, Canada. Physical characteristics of the water column, e.g. circulation and stratification, changed seasonally resulting from variations in tides, temperature, precipitation and wind. Chemical characteristics, e.g. oxygen and dissolved nutrient concentrations, also differed temporally relative to those in the lagoon's ocean source water (Juan de Fuca Strait) because of variations in local photosynthesis and nutrient use by phytoplankton. Diatom blooms occurred in the spring, and blooms of photosynthetic flagellates (Heterosigma akashiwo (2009) and Akashiwo sanguinea (2009 and 2010)) occurred in the late summer and early fall when nitrate, ammonium, and urea were depleted. Proliferation of these flagellates led to the development of harmful algal blooms (HABs) associated with oxygen depletion in the lagoon bottom waters. Increased oxygen demand from bacterial degradation of algal biomass and exudates was the likely cause for bottom water hypoxia under reduced tidal exchange.;During the spring phytoplankton bloom, maximum biomass was 27 mug L -1 chlorophyll a. Diatoms dominated the phytoplankton assemblages from March to mid-June. During this period dissolved nutrients were depleted from high winter concentrations to the lowest levels observed in this study. Nitrate, ammonium, urea, and orthophosphate decreased from 25.6 mumol L-1, 6.7 mumol L-1, 0.31 mumol L-1, and 2.2 mumol L-1, respectively, to near limits of detection, and silicic acid decreased from 52.7 mumol L -1 to 5.8 mumol L-1.;Blooms of photosynthetic flagellates occurred from August through October in 2009 and August through September in 2010. The maximum biomass achieved during these blooms was 30 mug L-1 chlorophyll a. All forms of nitrogen remained depleted throughout this latter part of the growing season, although orthophosphate concentrations were intermediate and silicic acid was abundant.;In diatom-dominated assemblages, rates of dissolved nitrogen uptake (sum of nitrate, ammonium, and urea uptake) correlated strongly with rates of dissolved inorganic carbon uptake, but in photosynthetic flagellate-dominated assemblages, the correlation was poor. This suggests that photosynthetic flagellates were utilizing other forms of dissolved or particulate organic nitrogen to fulfill their nitrogen requirements when nitrate, ammonium and urea were low.;Oxygen depletion developed in bottom waters of the lagoon during blooms of photosynthetic flagellates, and zones of hypoxia were observed in August of 2009 and September of 2010. Senescent phytoplankton cells and phytoplankton exudates, such as microsporine-like amino acids, are labile forms of organic matter that can be produced during the decline of large blooms, stimulating bacterial growth and increasing oxygen demand. Microsporine-like amino acids were measured in 2010 and found to be present during the A. sanguinea bloom.;A critical factor that may have contributed to the persistence of low oxygen conditions and hypoxia was the narrowing of tidal ranges in the late summer and early fall compared to earlier in the growing season. Narrow tidal ranges reduce tidal current velocities and flushing rates.;This study demonstrates that understanding HAB development requires characterization of both phytoplankton bloom dynamics in the target system and the physicochemical processes that can affect bloom dynamics and intensify oxygen depletion.