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Spatial and temporal distributions of planktonic diatoms in a subtropical bayou, along the upper Texas coast.

Article Excerpt
Abstract.-A study of planktonic diatom (Bacillariophyceae) community spatial and temporal distributions was carried out in Offatts Bayou, a small embayment within the larger Galveston Bay complex in southeast Texas. The dominant diatom genera were Chaetoceros, Ditylum, Rhizosolenia, Coscinodiscus, Guinardia, Dactyliosolen, Odontella and Lithodesmium, in this order. Over 20 other diatom genera were represented at different times including some known to produce toxins (e.g., Pseudonitzschia), while others only appeared in surface waters after strong wind induced mixing events (e.g., Navicula). The spatial and temporal patterns observed for the dominant planktonic diatoms followed changes in salinity and temperature. Other environmental variables (light, nutrients, grazing) were not measured as part of this initial investigation but may have been important. Mean Chaetoceros, Ditylum, Odontella and Lithodesmium standing crops were highest at the open end of the bayou, while Guinardia and Dactyliosolen numbers were highest in the most flow-restricted regions of the bayou. Spatial distributions of Rhizosolenia and Coscinodiscus were homogenous throughout Offatts Bayou. Understanding natural diatom succession patterns may provide important insights into normal variations in community composition. Diatoms are potentially a powerful biomonitoring tool for future assessments of the impacts of eutrophication, climate change and/or human induced ecosystem dysfunction.

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Climate change and sea level rise will pose complex effects on phytoplankton communities, particularly those in coastal environments. Eutrophication induced decline in ecosystem function in coastal areas has resulted in algal blooms, red and/or brown tides (Hallegraeff 1993; Nixon 1995; Howarth et al. 2000), fish disease, fish kills, and the development of hypoxic areas (Paerl et al. 1998; Thronson & Quigg 2008). Like bays and estuaries, coastal bayous are especially susceptible to contamination because they have intrinsically low flushing rates and limited freshwater inflow. In their tidal reaches, bayous are high in suspended sediments and dissolved organic carbon, and low in dissolved oxygen, especially from late spring to early fall (e.g., Dickinson Bayou, Texas; Quigg et al. 2009).

Given that diatoms (Division Bacillariophyta) are important primary producers in coastal waters, making up a large fraction of the phytoplankton community in many places (Ornolfsdottir et al. 2004a; Ornolfsdottir et al. 2004b; Bukyates & Roelke 2005; Lavoie et al. 2006), changes in their dynamics may be used as a sensitive biomonitor of the influence of the above processes on ecosystem function. Diatoms respond quickly to physical, chemical and biological perturbations (e.g., Ornolfsdottir et al. 2004a; Ornolfsdottir et al. 2004b). Yet few studies have examined diatom community composition and succession in coastal systems (Resende et al. 2005; Cetinic et al. 2006) despite growing concerns for ecosystem health in these, the most densely populated, developed and growing areas in the world.

Galveston Bay (Fig. 1) is the second largest estuary (ca. 1550 [km.sup.2]) on the Texas coast. It faces conservation issues due to high density industrialization and urbanization throughout its watershed (GBEP 2001; TWDB 2007). A human population of four million live within its five bordering counties; this figure doubles when considering the entire watershed which encompasses two of the largest metroplexes in Texas: Houston and Dallas-Fort Worth (TWDB 2007). Galveston Bay is the most productive of all Texas' estuaries with an oyster production that is unsurpassed in the U.S., and a recreational fishery that is worth millions of dollars annually (GBEP 2001; Lester & Gonzalez 2002; TWDB 2007).

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Phytoplankton biomass (measured as chlorophyll a) has been declining in Galveston Bay since the 1970's (Lester & Gonzalez 2002). The bay now sustains a moderate to low phytoplankton biomass of 4 to 15 ug chlorophyll a [L.sup.-1] (Santschi 1995; Quigg et al. 2007). The decrease is thought to be a response of the bay to improved water quality as a result of the Clean Water Act of 1970 (H-GAC 2006). Concurrent decreases in nitrogen and phosphate loading have also been recorded (Santschi 1995; Lester & Gonzalez 2002) along with increased water clarity (Lester & Gonzalez 2002). Although phytoplankton blooms occur at different times of the year in the bay, they typically do not reach proportions which may be considered 'harmful'. The appearance of the diatom Nitzchia pungens forma multiseries in 1989 in Galveston Bay (Fryxell et al. 1990) and in 1990 in Offatts Bayou (Reap 1991) first raised concerns for human health and for the local fisheries nearly two decades ago. The presence of this and other diatoms (and speciesof other phytoplankton) capable of producing toxins remains an issue of concern for scientists, resource managers and residents.

In order to develop an effective biomonitoring tool, it must...

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