Summer algal blooms in shallow estuaries: Definition, mechanisms, and link to eutrophication

Abstract

We propose a definition for identification of blooms and use this definition to investigate the underlying mechanisms of summer blooms and their link to nutrient enrichment. Blooms were defined as chlorophyll a observations deviating significantly from a normal seasonal cycle; the frequency and magnitude of these deviating observations characterized bloom frequency and intensity. The definition was applied to a large monitoring data set from five estuaries in Denmark with at least biweekly sampling. Four mechanisms with links to nutrient enrichment were identified as sources of summer blooms: (1) advection from biomass-rich inner estuary, (2) resuspension of nutrients and algae from sediments, (3) nutrient releases from sediments during hypoxic conditions, and (4) decoupling of benthic grazers. Summer blooms were mostly dominated by diatoms, and in 33% of the bloom samples the dominating species was also dominant prior to the bloom. Only four species (Cerataulina pelagica, Chaetoceros socialis/radians, Prorocentrum micans, and Prorocentrum minimum) typically (.50% of blooms) increased their biomass proportion during bloom initiations. Bloom frequency and intensity decreased from 1989 to 2004, corresponding to decreases in nutrient inputs and concentrations, but only bloom frequency could be directly linked to the actual total nitrogen concentrations, whereas bloom intensities depended on site-specific features, particularly a threshold response for stations exposed to hypoxia. Bloom frequency has increased over longer timescales in response to nutrient enrichment.

Temporal Coherence of Chlorophyll a during a Spring Phytoplankton Bloom in Xiangxi Bay of Three‐Gorges Reservoir, China

“…The demonstration of such synchrony for limnological variables from those studies has important implications in understanding long-term trends of lakes ecosystem and theoretical and practical significance in improving long-term management of lakes ecosystem. However, the detection of temporal coherence based on low sampling frequencies fails to explain spatial variation in short term lake dynamics such as algal bloom phenomenon, which was described as "the rapid growth of one or more phytoplankton species which leads to a rapid increase in the biomass of phytoplankton" (CARSTENSEN, 2007). Considering the rapid growth of phytoplankton during bloom period, the present study illustrated temporal coherence for chlorophyll a based on higher sampling frequencies (weekly monitoring) than previous studies.…”

Interannual variation in submerged aquatic vegetation and its relationship to water quality in subestuaries of Chesapeake Bay

“…One variable, May chl a, is indicative of light availability in May when oligohaline SAV germinate and establish and are therefore especially vulnerable to disturbance. High levels of chl a can block light from reaching SAV, especially during dense algae blooms , Smayda 1997, Carstensen et al 2007. Although blooms are typically short-lived, they can be devastating to young SAV (Gallegos & Jordan 2002), as documented in a well-studied May 2000 mahogany tide Prorocentrum minimum bloom that decimated upper Chesapeake Bay SAV (Gallegos & Bergstrom 2005).…”

Strength and uncertainty of phytoplankton metrics for assessing eutrophication impacts in lakes

“…Cyanobacteria are widely recognised to increase in dominance and abundance in response to increasing nutrient concentrations, often resulting in dense, mono-specific blooms during summer in eutrophic waters Watson et al, 1997). Lake ecologists also use the term "bloom" to refer to spring and autumn increases in diatoms (Reynolds, 1984) and marine biologists refer to blooms of diatoms or dinoflagellates (Carstensen et al, 2007). Annex V of the WFD characterises moderate status lakes as those in which "persistent phytoplankton blooms" may occur during summer months and, for this reason, almost certainly had in mind summer blooms of cyanobacteria.…”