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.A. Moore, R.L. Wetzel J. Exp. Mar. Biol. Ecol. 244 2000 1 –28
1. Introduction
The declines of seagrass e.g., Zostera marina L., eelgrass and other submersed vascular plant communities worldwide have been attributed to deteriorating habitat
conditions den Hartog and Polderman, 1975; Orth and Moore, 1983; Silberstein et al., 1986. Nutrient enrichment can promote phytoplankton growth Phillips et al., 1978;
Boynton et al., 1982; Sand-Jensen and Borum, 1991 which increases turbidity Dennison et al., 1989, and runoff of suspended sediments and dissolved substances
further increases water column light attenuation, especially in estuarine areas Champ et al., 1980; Kemp et al., 1983; McPherson and Miller, 1987; Moore et al., 1997.
Decreased light availability can have adverse effects on Z
. marina photosynthesis Dennison and Alberte, 1982, growth Dennison and Alberte, 1985; Dennison, 1987,
community structure Backman and Barilotti, 1976; Short et al., 1995, and ultimately long-term survival Zimmerman et al., 1995; Moore et al., 1996. With increased
nutrient loading, seagrasses have been replaced by macroepiphytes in some systems Duarte, 1995; Coffaro and Bocci, 1997; Valiela et al., 1997. Nutrient enrichment has
also been related to increased epiphytic growth on macrophyte shoots in the field Cambridge and McComb, 1984; Borum, 1985. Epiphytes are thought to reduce plant
growth by reducing light availability as well as limiting bicarbonate uptake for photosynthesis Sand-Jensen, 1977. Additionally, there is some evidence that elevated
concentrations of water column nitrate may be toxic to some submersed macrophytes Burkholder et al., 1992, 1994.
Survival of submersed plants is dependent on the interactions of many biotic and abiotic factors that affect the balance between production and respiration Zimmerman et
al., 1995. Little is known, however, about the combined influences of many of these factors that operate simultaneously in nature. For example, in many areas, such as the
York River in Virginia where seagrasses have declined, both water column turbidity and nutrients are greater in impacted sites than unimpacted sites Batuik et al., 1992; Moore
et al., 1996. Although such field observations can be used to infer the possible linkages between habitat conditions and macrophyte survival Dennison et al., 1993, they
provide only correlative information. Controlled experiments are necessary to define the causal relationships between multiple controlling factors and seagrass growth e.g.,
Barko et al., 1991; Neckles et al., 1993.
Many nutrient enrichment studies on macrophytes attempt to simulate nutrient loadings to a system using pulsed inputs. Usually water turnover is low and initial levels
of enrichment are very high, ranging up to 100 3 of controls, followed by rapid decreases in water column concentrations with time Burkholder et al., 1992; Neundorfer
and Kemp, 1993. However, field observations suggest that rapid turnover of water combined with constant but lower increases in water column nutrient concentrations
e.g., 2 3 to 3 3 ; Neckles et al., 1993 are more characteristic of differences between many regions where Z
. marina has survived and areas where it has died out Batuik et al., 1992; Moore et al., 1996.
In addition, while experiments have demonstrated that long-term light reductions can significantly impact Z
. marina communities Backman and Barilotti, 1976; Short et al., 1995, in natural systems relatively short-term periods of high turbidity may be limiting
K .A. Moore, R.L. Wetzel J. Exp. Mar. Biol. Ecol. 244 2000 1 –28
3
Z . marina survival Zimmerman et al., 1995 and seasonal reductions of 30 to 40 days in
duration are sufficient to limit Z . marina survival Dennison and Alberte, 1985; Moore
et al., 1997. In this study, the seasonal responses of Z
. marina and its epiphyte community to decreased water column light availability and increased nutrient concentrations under
conditions of rapid water turnover were investigated. The objective was to determine the relative importance of these two environmental factors which have been related to Z
. marina declines and to determine their single and interactive effects on Z
. marina communities in experimental microcosms.
2. Methods
