Abstract
BackgroundMarine angiosperms, seagrasses, are sentinel species of marine ecosystem health and function. Seagrass carbon budgets provide insight on the minimum requirements needed to maintain this valuable resource. Carbon budgets are a balance between C fixation, growth, storage and loss rates, most of which are well characterized. However, relatively few measurements of dissolved organic carbon (DOC) leaf exudation or rhizodeposition rates exist for most seagrass species. Here I evaluate how eelgrass (Zostera marina L.) DOC exudation is affected by a single factor manipulation (light, temperature or salinity). Eelgrass plants were hydroponically exposed to treatments in experimental chambers (separate leaf and rhizome/root compartments) with artificial seawater medium. Regression analysis of changes in the DOC concentration through time was used to calculate DOC exudation rates.ResultsExudation rates were similar across all treatments in all experiments. For all experiments, pooled leaf DOC exudation ranged between 0.032 and 0.069 mg C gdw-1 h-1, while rhizodeposition ranged between 0.024 and 0.045 mg C gdw-1 h-1. These rates are consistent with previously published values and provide first-order estimates for mechanistic models.ConclusionsZostera marina carbon losses from either leaf exudation or rhizodeposition account for a small proportion of gross primary production (1.2-4.6%) and appear to be insensitive to short-term (e.g., hours to days) environmental variations in chamber experiments. Based on these preliminary experiments, I suggest that Z. marina DOC exudation may be a passive process and not an active transport process.
Highlights
Marine angiosperms, seagrasses, are sentinel species of marine ecosystem health and function
Effect of light intensity There were no significant differences among dissolved organic carbon (DOC) exudation rates for experimental light treatments either for leaf (ANOVA df = 2, F = 1.84, P = 0.213) or rhizome and root (RR) (ANOVA df = 2, F = 0.02, P = 0.975) tissue
Mean leaf DOC exudation rates accounted for about 1.4% of gross primary production (GPP), while RR exudation accounted for ca. 1.5% of GPP (Table 3)
Summary
Seagrasses, are sentinel species of marine ecosystem health and function. Carbon budgets are a balance between C fixation, growth, storage and loss rates, most of which are well characterized. Relatively few measurements of dissolved organic carbon (DOC) leaf exudation or rhizodeposition rates exist for most seagrass species. Seagrasses are marine angiosperms that provide valuable ecosystem services and are often described as foundation species or ecosystem engineers [1,2] They have been identified as sentinel species that can indicate marine ecosystem health and function [3]. Seagrass carbon budgets like those of all plants are a balance between C fixation, growth, storage and loss rates, in the simplest form: WP 1⁄4 Cfixed À Lfresp À Lfexud À RRresp À RRexud À Structural ð1Þ. Pioneering work conducted during the late 1970’s and early 1980’s suggested that carbon loss via exudation (DOC, dissolved organic carbon) from leaves was small [13,14,15]. The contrasting conclusions from these studies and lack of work taking into account variability in environmental conditions suggest that further attention is required to better understand and model these processes
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