Particulate organic matter deposition and sediment flux in and around seagrass meadows along the Bellarine Bank in Port Phillip Bay

Large-scale historical contraction and expansion of seagrass meadows in Port Phillip Bay (PPB) has been well documented. Although theories outlining potential drivers have been posited, no definitive mechanism has been... [ view full abstract ]

Large-scale historical contraction and expansion of seagrass meadows in Port Phillip Bay (PPB) has been well documented. Although theories outlining potential drivers have been posited, no definitive mechanism has been described despite the importance of seagrass meadows to the health of the PPB ecosystem. This study formed part of a larger study into causes of seagrass meadow contraction and mechanisms supporting resilience. We hypothesised that; i) in nutrient poor environments, trapping of particulate organic matter (POM) by seagrass would constitute a significant mechanism for increasing resilience through nutrient capture, and ii) seagrass burial can be a significant mechanism for reduction in cover in medium and high energy environments. We investigated the influence of seagrass patch size and position on current attenuation, POM trapping and sediment flux rates (deposition minus resuspension) along the Bellarine Bank. We also measured seagrass community productivity rates, and temporal variation in seagrass cover at these sites.
At Clifton Springs in off shore seagrass meadows (2-2.5m depth), the presence of seagrass patches increased average vertical current attenuation and POM delivery compared to bare sand sites. Variation in patch size did not influence either current attenuation or POM capture rates. POM deposition rates equate to estimated nitrogen loading rates which were more than sufficient to support measured seagrass community productivity rates. Sediment nitrogen and carbon stable isotope signatures indicated that the predominant input of POM to seagrass patches was phytoplankton detritus. Sediment flux was overall depositional within seagrass patches and erosional at bare sand sites. Large-scale seagrass burial was not evident, instead seagrass patches waxed and waned over annual timescales within troughs between sand ridges and seagrasses migrated with ridges at a rate of 4 -5 m.y-1.
In a near shore seagrass meadow (0.1–1.5m depth) at Point Richards, POM deposition rates were tied to sediment deposition rates, which were high due to imported sediment loads associated with strong easterly currents. These high rates of sediment deposition appear to have caused recent loss of seagrass meadows due to burial, and possibly also through raising seagrass meadows up to a higher wave energy zone at shallower depths.