Potential for landscape-scale positive interactions among tropical marine ecosystems

Fluxes of energy, materials and organisms among ecosystems are consequences of their openness to exchange and lead to the consideration of reciprocal connections among adjacent ecosystems. Reciprocal connectivity may have implications for ecosystem functioning and management but it is generally studied only for a single factor, rather than for multiple factors. We examined the extent to which these fluxes may apply at the landscape scale for 3 ecosystems: mangrove forests, seagrass beds and coral reefs. From a literature review and analysis, we semi- quantitatively assessed fluxes based on attenuation of wave height and exchanges of sediments, nutrients and algivores. We found that coral reefs and seagrass beds significantly attenuated wave height and that this effect depended on specific physical conditions. In the case of coral reefs, the attenuation capacity depended on the section of the reef the wave breached, whilst for seagrass beds, we hypothesised that the density of the plants was the controlling factor. Mangrove forests’ ability to reduce sediment fluxes was related to the mangrove forest area. Seagrass plants have a capacity to decrease sediments in the water column. Both mangrove forests and seagrass beds retained nutrients within the ecosystems, which was a positive interaction. Isolated reefs showed a decrease (30 to 95%) in algivore biomass compared to situations where the 3 habitats were in proximity to each other. The findings show that there is potential for reciprocal connections among coastal ecosystems. Our results indicate that these exchanges at the ecosystem scale can be placed into the context of facilitation in the field of community ecology. Future research should focus on which natural and anthropogenic factors determine reciprocal facilitation between these ecosystems and determine how ecosystem-based management can be improved with this knowledge. The considerable potential for reciprocal facilitation implies that ecosystem managers may need to place greater emphasis on the landscape scale.

L.G. Gillis, T.J. Bouma, C.G. Jones, M.M. van Katwijk, I. Nagelkerken,

C.J.L. Jeuken, P.M.J. Herman , A.D.Ziegler

Leaf transport in mimic mangrove forests and seagrass beds

Mangrove forests and seagrass beds are thought to exchange particulate organic material, especially in the form of leaves. However, relatively little is known about the trapping capacity of mangrove above-ground roots and seagrass plants for leaf segments. We aimed to identify the major factors controlling the leaf-trapping capacity of mangroves and seagrasses in a flume study. For mangroves, we found that higher density mangrove roots enhanced trapping capacity whereas the presence of waves strongly reduced the trapping capacity. The latter might be explained by a reduced average collision time (i.e. the time a leaf was attached to a root structure) in the presence of waves. The ability for seagrass beds to trap leaves was dominated by the length/type of vegetation and the absence/presence of waves. Overall, our results suggest that mangroves—via their roots—have a more efficient trapping mechanism than seagrass beds. Mangrove roots extend through the entire water column the majority of the time, which enhances trapping capacity. In contrast, seagrass beds require particulate organic material to become entangled within the predominantly submerged shoots, making trapping dependent on the degraded state of the leaf and the water depth. Our results give an indication of parameters which could be used in a model of trapping capacity of these ecosystems. As leaves are associated with nutrients, we have identified factors which will help to determine what parameters affect the nutrient retention or export of ecosystems. These include density of roots, hydrodynamic conditions (absence/presence of waves), location of initial deposition, degradation stage and type of leaf.

L.G. Gillis, T.J. Bouma, W. Kiswara, A.D. Ziegler, P.M.J. Herman

Tiny Is Mighty: Seagrass Beds Have a Large Role in the Export of Organic Material in the Tropical Coastal Zone

Ecosystems in the tropical coastal zone exchange particulate organic matter (POM) with adjacent systems, but differences in this function among ecosystems remain poorly quantified. Seagrass beds are often a relatively small section of this coastal zone, but have a potentially much larger ecological influence than suggested by their surface area. Using stable isotopes as tracers of oceanic, terrestrial, mangrove and seagrass sources, we investigated the origin of particulate organic matter in nine mangrove bays around the island of Phuket (Thailand). We used a linear mixing model based on bulk organic carbon, total nitrogen and d13C and d15N and found that oceanic sources dominated suspended particulate organic matter samples along the mangrove-seagrass-ocean gradient. Sediment trap samples showed contributions from four sources oceanic,

mangrove forest/terrestrial and seagrass beds where oceanic had the strongest contribution and seagrass beds the smallest. Based on ecosystem area, however, the contribution of suspended particulate organic matter derived from seagrass beds was disproportionally high, relative to the entire area occupied by mangrove forests, the catchment area (terrestrial) and seagrass beds. The contribution from mangrove forests was approximately equal to their surface area, whereas terrestrial contributions to suspended organic matter under contributed compared to their relative catchment area. Interestingly, mangrove forest contribution at 0 m on the transects showed a positive relationship with the exposed frontal width of the mangrove, indicating that mangrove forest exposure to hydrodynamic energy may be a controlling factor in mangrove outwelling. However we found no relationship between seagrass bed contribution and any physical factors, which we measured. Our results indicate that although seagrass beds occupy a relatively small area of the coastal zone, their role in the export of organic matter is disproportional and should be considered in coastal management especially with respect to their importance as a nutrient source for other ecosystems and organisms.

Lucy G. Gillis, Alan D. Ziegler, Dick van Oevelen, Cecile Cathalot,

Peter M. J. Herman,Jan W. Wolters, Tjeerd J. Bouma