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

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. Phytoplankton blooms, particularly harmful algal blooms (HABs), are believed to have expanded globally in coastal waters, although there are few long-term data sets available to critically evaluate this hypothesis (Smayda 1990; Hallegraeff 1993; Cloern 2001). Phytoplankton blooms are natural phenomena that were also occurring during pristine conditions (Bianchi et al. 2000), but it has become a widespread belief that the increasing frequency of blooms is related to anthropogenic nutrient enrichment of coastal waters, although this has not been proven rigorously (Paerl 1988, 1997; Cloern 2001). For example, Dale et al. (1999) observed increasing and decreasing trends in the abundance of dinoflagellate cysts corresponding to the signals of nutrient loading for the inner Oslofjord, and Carstensen et al. (2004) documented that interannual variations in the summer bloom frequency for the Kattegat were related to the nitrogen inputs. Particularly, the importance of atmospheric deposition (AD) as a source of ‘new’ nitrogen has been suggested as a primer for algal blooms (Paerl and Whitall 1999), although the relative importance of AD-N decreases with increasing mixing depth (Carstensen et al. 2005). Nutrient enrichment enhancing new production (Nixon 1995) may show as increased frequency and magnitude of phytoplankton blooms, but ecosystem attributes can act as a filter to modulate this response (Cloern 2001). This could explain why supporting evidence of the causative link to nutrient enrichment is still lacking. A bloom is generally perceived as a significant increase in biomass, meaning there is an unbalance between phytoplankton growth and loss processes. Blooms are uncoupled from grazing and normally terminated by sedimentation (Kiørboe et al. 1996). In many cases, blooms are beneficial to food-web processes (typically ‘‘miniblooms’’), whereas excess growth and sedimentation of relatively ungrazed species, such as Ceratium, may contri1 Corresponding author: [email protected]. Acknowledgments We thank the Danish Counties responsible for data collection under the Danish Nationwide Aquatic Monitoring and Assessment Program, and Department of Wind Energy, Risø National Laboratory and Sund & Bælt Holding A/S for providing wind data. Constructive comments from two anonymous reviewers improved the manuscript. This work was supported by two grants from the European Union (REBECCA SSPI-CT-2003-502158 and Thresholds GOCE-003933) and DNAMAP. Limnol. Oceanogr., 52(1), 2007, 370–384 E 2007, by the American Society of Limnology and Oceanography, Inc.