Seagrasses are marine angiosperms that provide valuable ecosystem services and are often described as foundation species or ecosystem engineers [1
]. Additionally, they have been identified as sentinel species that can indicate marine ecosystem health and function [3
]. Therefore, there is interest in using seagrass models to evaluate physiological and ecological effects of stressors such as nutrient loading, light reductions and geochemical toxicity (e.g.
, sulfides, nitrogen toxicity). Quantitative models focused on the plants’ carbon budgets provide detailed insight into their potential survival as it relates to varying levels of environmental stress [4
Seagrass carbon budgets like those of all plants are a balance between C fixation, growth, storage and loss rates, in the simplest form:
whole plant, Cfixed
photosynthesis, Lf and RR represent leaf and rhizome/root tissues, while subscripts resp
represent respiration and exudation loss terms respectively. Structural materials represent carbohydrates incorporated into cell walls during growth and development. A positive WP indicates surplus carbon that can be stored, while a negative WP indicates a carbon deficit that may be supplied from stored reserves [5
]. Many studies have focused on the fixation portion of carbon budgets especially with development of commercially available equipment and concomitant cost reductions to measure photosynthetic physiology using oxygen evolution [6
] and pulse amplitude modulated (PAM) fluorometry [9
]. Likewise, understanding carbon storage dynamics (e.g.
, non-structural carbohydrate carbon) provides insight into seagrasses stress tolerance, especially low light stress [12
]. 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
]. Recent work using compound specific stable isotope analyses could not detect coupling between Z. marina
production and sediment bacteria [16
] suggesting limited carbon exudation. As a result most seagrass studies and models neglect leaf DOC exudation. However, other recent work in tropical and subtropical seagrass systems suggests that DOC exudation can be substantial [17
]. 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.
Seagrass rhizodeposition, release of carbon exudates through rhizomes and roots, is thought to be a relatively minor loss [14
] and it has been generally ignored in seagrass production models. However, in some seagrass species, rhizodeposition is greater than leaf exudation and can account for 15-30% of primary production [18
]. Rhizodeposition is used synonymously with Rhizome
Root exudation throughout this document. In terrestrial plants, rhizodeposition can account for up to 17% of primary production and has been shown to fuel soil microbial processes [22
]. Likewise, a recent seagrass modeling study found that DOC rhizodeposition rates were a critical parameter for modeling microbially mediated sediment oxygen demand in a subtropical system [23
]. Although several studies have estimated DOC exudation and rhizodeposition they have been conducted under static environmental conditions. Variations in exudation rates under fluctuating environmental conditions or across a gradient of conditions may be important constraints for dynamic seagrass production models.
A number of studies have concluded that seagrass derived DOC contributes to the labile autochthonous carbon pool available to heterotrophic bacteria [24
]. Quantifying seagrass DOC production has generally been carried out using chambers to measure DOC fluxes from intact communities [13
]. These studies are inherently confounded by DOC exudation from multiple primary producer sources, including microalgal epiphytes, sediment microbial community (which may be heterotrophic or autotrophic) and water column planktonic and microbial communities as well as seagrass production. Using a variety of methods and assumptions, seagrass contribution to DOC efflux can sometimes be partitioned out. However, few if any studies have directly measured seagrass DOC production rates in vitro
by minimizing the influence of confounding primary producers (e.g.
, hydroponic chamber experiments), which will have their own unique limitations and caveats. Additionally, there have been no studies that evaluate how DOC loss rates respond to drivers that influence seagrass production.
Light, temperature and salinity are environmental drivers which have potentially large effects on carbon budgets by influencing rate processes and ultimately carbon balance. I predict that seagrass DOC exudation rates will be a function of these environmental drivers. My objectives were to develop a hydroponic chamber system for minimizing the number of DOC sources and to quantify how seagrass DOC exudation and rhizodeposition varied in response to a range of values for single environmental drivers (light, temperature or salinity).