DOC dynamics in a Mediterranean seagrass system

In a study on energy flow in a Posidonia oceanica system, an attempt was made to explain the marked seasonal variations in DOC, POM and bacteria concentrations in the water above and within a seagrass meadow at the Island of Ischia (Lacco Ameno) off the Gulf of Naples (Italy). There was no correlation between seasonal variations of the 3 parameters within and between stations from shallow to deep. DOC concentrations showed distinct peaks for nearly every sampling period and ranged from 0.4 to 31.4 mg 1-l. Although similar variations could be recorded for bacteria over the year, densities remained below 1.2 X 105 cells ml-l. Variations in the concentration of POM over the seasons were less obvious than for DOC and bacteria; concentrations ranged from 3.5 to 30 mg AFDW 1-' Weighted average concentrations of DOC, bacteria and POM in 1 m3 water representative for the water body above the P. oceanica bed showed that bacterial contribution to the overall POM standing stock is insignificant. Daily variations of the same parameters reveal peaks of DOC appearing first within the meadow, later in the water above the meadow, and a slow increase and decrease of bacteria and POM, following the DOC variations with a time lag. Deviations from this pattern could b e recorded as conditions of water movement changed, due to the mixing effect of wind-induced wave action. Bell-jar experiments with enclosed P oceanica shoots within the meadow and control jars enclosing bare sediment showed high DOC pulses up to 25 mg I-', indicating that most of the DOC in the water column may be generated by the P. oceanica system, and that allochthonous influx of carbon need not take place to explain the monitored seasonal and daily variations. Leaching from healthy P. oceanica leaves, determined in laboratory experiments, amounts to 1.9 % of the photosynthetic carbon fixation and contributes only 0.61 % of the system's DOC release. amounting to 571.9 mg C h-' m-2. The organic carbon pool in the sediment, expressed as particulate and dissolved fraction, shows that for temperature conditions between April and September the release rates, as determined by the in situ experiments, could be maintained for 157 d, assuming that no new carbon influx takes place. The fate of the released DOC is discussed, and since high DOC decrease rates could not be correlated to high respiration rates indicating heterotrophic consumption, nor to biomass increase of bacteria, physicochemical processes must take place which result in a phase shift from DOM to POM.