Abstract
Estuaries are highly productive and ecologically rich areas that are important habitats for fish and bird species. Over the past few decades, the frequency and duration of harmful algal blooms (HABs) have been increasing globally in coastal areas.1 HABs, especially those caused by the red tide organism, Karenia brevis, occur frequently along Florida's west coast, causing episodes of high mortality in fish, sea turtles, birds, bottlenose dolphins and manatees.2-4 Although red tide is known to cause episodes of mass mortality among marine vertebrates, it is not known whether this disturbance results in significant declines in bird populations or changes in community structure. We investigated the extent to which red tide affected the local abundance and habitat use of piscivorous birds in the Sarasota Bay estuary. Data were obtained by conducting boat-based seasonal surveys of birds, K. brevis cell counts, and water quality (dissolved oxygen, salinity, temperature, turbidity) during red tide and non-red tide conditions. Summer and winter surveys were conducted in four habitats (mangrove fringe, open bay, sandflat and seagrass) between June 20, 2006 and Sept 2, 2009. Over 36,000 bird observations were obtained involving over 20 different species. The most abundant bird species were double-crested cormorants (Phalacrocorax auritus), laughing gulls (Larus atricilla), and brown pelicans (Pelecanus occidentalis). Certain bird species were only counted in the winter seasons included migrating species such as the American white pelican (Pelecanus erythrorhynchos), common loon (Gavia immer), and red–breasted merganser (Mergus serrator). Periods of high K. brevis concentrations (> 105 cell/l) occurred during the summer 2006 and the winter 2007 seasons. Overall bird densities were lower during red tide conditions than they were during non-red tide conditions. In particular, the abundance of double-crested cormorants decreased in all habitats during red tide conditions, and rebounded by 2008. In contrast, brown pelicans and laughing gulls increased in abundance in certain habitats during red tide conditions and rebounded to lower abundances by 2008. Kreuder et al.3 found that double-crested cormorant strandings increased during red tide conditions. It is probable that cormorants are consuming different prey than pelicans and gulls and may be exposed to a higher dose of toxin leading to increased morbidity and mortality, and lower abundances during red tide events.
References
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2. Flewelling LJ, Naar JP, Abbott JP, Baden DG, Barros NB, Bossart GD, Bottein MYD, Hammond DG, Haubold EM, Heil CA, Henry MS, Jacocks HM, Leighfield TA, Pierce RH, Pitchford TD, Rommel SA, Scott PS, Steidinger KA, Truby EW, Van Dolah FM, Landsberg J.H. Brevetoxicosis: Red tides and marine mammal mortalities. Nature 2005; 435: 755–756.
3. Kreuder C, Mazet JAK, Bossart GD, Carpenter TE, Holyoak M, Elie MS, and Wright SD. Clinicopathologic features of suspected brevetoxicosis in double-crested cormorants (Phalacrocorax auritus) along the Florida gulf coast. Journal of Zoo and Wildlife Medicine 2002; 33: 8–15.
4. Landsberg JH, Flewelling LJ, Naar J. Karenia brevis red tides, brevetoxins in the food web, and impacts on natural resources: Decadal advancements. Harmful Algae 2009; 8: 598–607.