Annual metabolic carbon balance of the seagrass Posidonia oceanica: the importance of carbohydrate reserves

Abstract

We present the seasonal carbon (C) balance of the Mediterranean seagrass Posidonia oceanica (L.) Delile calculated from seasonal rates of C gain (photosynthesis), C loss (respiration) and growth. We compare our balance with the evolution of seasonal C reserves in order to determine the parameters (shoot:root biomass, reserve allocation, photosynthetic parameters, etc.) that influence the seasonal cycle of the plant. Additionally, we examine whether the annual C balance can be used as a valid tool for testing the vulnerability of seagrasses to light reduction. The seasonal whole-plant C balance showed alternate negative (from September to June) and positive (July and August) values. This trend was the result of the interplay among several seasonal factors such as irradiance, water turbidity, photosynthetic parameters, respiratory rates, shoot growth, within-shoot age distribution, and principally, the low photosynthetic:non-photosynthetic biomass ratio. The lack of significant correlation between seasonal growth and metabolic balance (C gain -C demand) did not permit the prediction of plant growth. Conversely, the seasonal pattern of carbon storage was consistent with the periods of positive and negative C balance. Consequently, reserve mobilization allows overwintering and re-growth under conditions of negative C balance. Using different calculations the annual C balance was found to be negative during 1993; this is in accordance with the carbohydrate interannual depletion and the shoot density decline. Since Posidonia oceanica is regressing in the Mediterranean, our carbon budget may notably contribute to future carbon models that can be essential tools for defining the minimum light requirements for survival. More insight into the functioning of some of the parameters that definitively influence this carbon budget (e.g.: the rhizome/root oxygen consumption and the O 2 to C conversion) is needed to fully understand the vulnerability of seagrasses to light reduction.

Thresholds of irradiance for seagrass Posidonia oceanica meadow metabolism

“…Leaf production rates were not significantly affected by our shading treatments, indicating that carbohydrate reserves in the rhizome might support leaf growth (Alcoverro et al 2001). This is in accordance with the lack of significant effects of shading on Posidonia oceanica meristematic activity.…”

“…), which in the most conservative scenario represents 40% of the maximum daily irradiance reached during the experiment and ~3-fold the I C estimated for single P. oceanica leaves at the same time of the year (Alcoverro et al 2001). When comparing our results with those from seagrass communities elsewhere, our estimates are similar to those quantified in subtropical systems, such as meadows of Thalasia testudinum from Northern Florida (I C from 340 μE m −2 s −1 to 107 μE m −2 s −1 ; Calleja et al 2006 andHerzka &Dunton 1997, respectively).…”

Combined nutrient and macroalgae loads lead to response in seagrass indicator properties

“…Although seagrasses can store enough carbohydrates to maintain metabolic processes and support seagrasses survival over a short period, light reduction, either or not due to macroalgae canopy may result in mortality over longer time spans Alcoverro et al, 2001;Brun et al, 2003). Some Ulva species can survive for 2 weeks in the dark (Kamermans et al, 1998).…”

Molecular composition of plant parts and sediment organic matter in a Mediterranean seagrass (Posidonia oceanica) mat

“…The root is often short but strong and grows adventitiously. The detritus of belowground organs (rhizomes, roots and sheaths), which constitute the vast majority of the plant's dry weight (Alcoverro et al 2001), are incorporated directly into the seabed, thereby protected against subsurface currents, wave action and herbivores. The aboveground organs (leaves), on the other hand, are largely eroded from the meadows and exported to adjacent systems including supralittoral environments forming the so-called banquettes and agaepropili 1 (Mateo et al 2002;Simeone and De Falco 2012).…”