Potential Impacts of Climate Change on Seagrass Productivity

The dramatic response to CO2 enrichment exhibited by a number of seagrass species indicates that carbon limitation of photosynthesis may be a common feature of these submerged angiosperms (Beer andWaisel, 1979; Millhouse and Strother, 1986;Du-rako, 1993; Abal et al., 1994; Invers et al., 2001). Although the absolute kinetics of photosynthesis vs.

[CO2] appear to be species specific (Invers et al.,

2001), the response generally involves a non-linear increase in photosynthesis in response to increasing [CO2], which yields a negative response to pH (a proxy for [CO2] in seawater, Fig. 10). This effect can be easily incorporated into the biophysical model by defining Pm as a function of pH{= f (— log[CO2])}. In the case of eelgrass, the increase in Pmax can be described by:

This relation sensitizes canopy productivity and daily carbon balance calculated by Eqs. (18) to (21) to CO2 availability. Using a similar approach to that described above for the analysis of water quality on eelgrass distributions in Dumas Bay, the potential impact of increased atmospheric [CO2] on eelgrass distribution was explored in Elkhorn Slough, California, USA, a moderately turbid estuary where eel-grass distributions are limited to less than 2 m depth (Zimmerman et al., 1994; Zimmerman and Caffrey,

2002). Patchy meadows currently occupy about 17 ha of shallow habitat along margins of the main channel of the Slough, and these realized distributions are consistent with model predictions of light-limited distribution based on average water quality conditions and bathymetry (Fig. 11). Assuming present day bathymetry and water quality, a doubled atmospheric CO2 concentration would increase the areal coverage of eelgrass in Elkhorn Slough to 23 ha, a 35% increase over the present distribution. In addition to increasing the density of existing beds, the model predicts eelgrass to colonize the basin at the top of Seal Bend, which is presently too deep for successful vegetation. Although atmospheric CO2 concentrations are not expected to rise to a level that prevent carbon limitation of seagrass photosynthesis, increasing the CO2 concentration to the point where seagrass photosynthetic performance is equivalent to that of marine algae (pH 6.2) may yield a six-fold increase in eelgrass coverage and greatly increase the density of existing vegetation.

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