Batan Bay and Tinago Lake are shallow embayments connected to each other, located on the north of Panay Island, central Philippines (11.53° – 11.67°N, 122.38° – 122.52°E). Although they had been originally surrounded by dense mangrove forest till the middle of the last century, mangroves have been mostly cleared and converted into fish and shrimp ponds. Recently, shelves and rafts for cultivating oysters and green mussels have become widespread in the shallow areas of the embayments (see Figure as an example). Replantation of mangroves is also ongoing in limited areas of Batan Bay. We are conducting researches there focusing on ecosystem services of mangroves and seagrass meadows, especially in relation to carbon sequestration and aquaculture production. In this presentation, we report preliminary survey results on environmental conditions that may influence growth and survival of cultivated bivalves, such as freshwater inputs and potential food resources. The survey was conducted in both dry season (February 2019) and rainy season (November 2019). Although the salinity gradient across the bay due to freshwater input was evident in both seasons, the oxygen isotope ratio of seawater indicated that evaporation overwhelmed in inner bay sites in the dry season. Concentrations of chlorophyll and suspended particulate organic matter (POM), i.e. potential food source for bivalves, were high in the inner bay area. Carbon stable isotope ratio (δ13C) of dissolved organic carbon (DIC) and POM showed spatial gradient from the bay mouth (high) to inner sites (low), indicating the influence of riverine DIC and POM inputs. However, the δ13C of oysters (adductor muscle) was consistently higher than POM and showed no clear spatial gradient. The δ13C of oysters was relatively higher for individuals collected from inside or edge of seagrass meadows than those collected in open areas. These results suggest that oysters assimilate only a specific fraction of POM relatively enriched in 13C (i.e. marine-origin POM) and that seagrass meadows support growth of oysters by providing additional food source (e.g. attached microalgae that are abundant on seagrass blades).

Mangrove forests with complex root systems contribute to increased coastal protection through drag effects. Previous flume studies proposed a predictive model of drag in Rhizophora mangrove forests based on quadratic drag law. However, its general applicability on mangrove forests in the field has not been tested. To fill this knowledge gap, this study quantified drag in a 17-year-old planted Rhizophora mangrove forest using a comprehensive measurement of hydrodynamics and vegetation morphology. The vegetation projected area density, a, showed an approximate exponential increase towards the bed, mainly due to root branching. This vertical variation led to enhanced vegetation drag per unit water volume relative to velocity with decreasing water depth. Alternatively, the drag per vegetation projected area solely depended on the square of velocity, indicating association with the quadratic drag law. The derived drag coefficient (CD) was 1.0 ± 0.2 for tide-driven currents, consistent with previous flume studies. By using the mean value of derived CD (1.0), it was confirmed that the quadratic drag model expresses well the field-measured drag. We also presented a method for predicting a value for a, another unknown parameter in the drag model, using an empirical Rhizophora root model, and confirmed a successful prediction of a and drag. Therefore, the drag in a Rhizophora mangrove forest can be accurately predicted only using the input parameters of the Rhizophora root model – stem diameter and tree density. This provides insights into effectively implementing the drag model in hydrodynamic models for better representation of mangroves’ coastal protection function.