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...California, previously unpublished data for the "normal year" of 1992-1993. We found significant differences for most of the variables measured, which included Leaf Area Index, leaf length, width, dry weight and area, biomass, shoot density, and number of leaves per shoot. Inspection of the multivariate ENSO index (MEI) and sea surface temperature (SST) anomalies for each of these years showed that the differences could be explained by the warm SSTs associated with the ENSO event. We were able to explain the observed differences from dynamic perspective by using a leaf-growth model forced by SSTs. We conclude that sea surface temperature summarizes the fundamental environmental influences on eelgrass leaf dynamics observed in our study site. That is, higher SSTs explain the reduction in mean leaf lengths and the corresponding diminution in related productivity variables. This study also strengthens the view that the onset of an El Nino event provides
anticipatory evidence for the effects that a rise in global temperature is expected to elicit in eelgrass beds.
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Human-induced increases in atmospheric concentrations of greenhouse gases are expected to exacerbate global temperature change (Ramanathan 1988). The associated alterations could be particularly deleterious for estuaries and coral reefs, currently under noticeable anthropogenic stress. These relatively shallow environments will be affected through alterations in abiotic variables like temperature and sea level, wind patterns and storminess, availability of precipitation water and runoff, and modifications in availability of nutrients (Dyer 1985; Emanuel 1987; Wigley and Raper 1987; Bakum 1990; Peterson et al. 1993; Watson et al 1996; Kennedy et al. 2002).
Sea-surface temperature has been found to have a great influence in marine ecosystem dynamics (Tegner and Dayton 1987; Baumgartner et al. 1992; Beer and Koch, 1996; Holbrook et al. 1997; Johnson et al. 2003). In particular, warmer temperatures are expected to influence the biology of organisms by reducing the concentration of dissolved oxygen in seawater. Diurnal and annual temperature oscillations also influence the biology and distribution of aquatic angiosperms (Holmes & Klein, 1987). Several workers have reported that growth dynamics of the temperate seagrass Zostera marina L. is highly correlated with sea surface temperature (Short an Neckles 1999; Solana-Arellano et al. 1997, Poumian-Tapia & Ibarra-Obando 1999; Solana-Arellano et al. 2004). According to Setchel (1929) eelgrass can grow only within a fixed temperature range. Other authors (Rasmussen 1977; Phillips and Backman 1983) have also shown that temperature is fundamentally important in controlling the seasonal growth cycle of eelgrass.
The distribution and abundance of seagrasses in temperate littoral waters are also controlled by light availability (Backman and Barilotti 1976; Denninson and Alberte 1985; Bulthuis and Woelkerling 1983; Orth and Moore 1988; Zimmerman et al 1991). In particular, light has been shown to influence the distribution (Dennison & Alberte 1985), density (Mukai et al. 1980), flowering (Phillips & Backman 1983), biomass (Mukai et al. 1980) and production (Bulthuis 1987) of Zostera marina.
Dissolved oxygen, inorganic nutrients (including carbon) and water movements also modify photosynthesis in aquatic plants. While light, temperature and dissolved oxygen regulate instantaneous photosynthetic rates, the availability of inorganic nutrients affects the long-term response of photosynthesis by controlling the levels of photosynthetic enzymes and pigments (Solana-Arellano et al. 1997). Eelgrass can absorb nutrients either from the roots or the leaves (McRoy et al. 1972). Hence, modifications in upwelling activity, stratification, and tidal dynamics could alter the availability of dissolved nutrients and thus affect seagrass productivity. In our study area (San Quintin Bay, Baja California, one of the last remaining ecologically functional estuaries in the California Bight), the onset of an ENSO event alters sea surface levels and temperatures, as well as tidal dynamics and nutrient availability (Alvarez-Borrego 2004). Hence, it is reasonable to expect that ENSO-like conditions would influence eelgrass dynamics in the bay. Sea surface temperature provides a reasonable paradigm for an ENSO-like alteration. Since thermal expansion of the water column influences sea level variation (Wigley and Rapper 1987) sea level can be assumed to increase, thus modifying light availability and changing tidal dynamics and circulation (Short and Neckles 1999), Also, thermally induced stratification would imply a deeper thermocline and nutricline, and modify upwelling transport of dissolved nutrients (Petterson et al. 1993). Hence we would expect sea surface temperature to be a variable that triggers changes in the dynamics of the principal environmental influences on eelgrass in our study site.
To provide a basis for the assumption of significant causal linkage between an ENSO warming event and changes in the growth dynamics of eelgrass we demonstrate here that sea surface temperature accounts in a fundamental way for the observed variability of standing stock variables by comparing two whole-year-cycle data sets of Z. marina L. in the San Quintin Bay estuary. The first data set, for the ENSO...
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