Aquatic ecosystems are difficult to manage owing to the myriad of ecological and physical processes driving these systems. Two complex yet fundamentally important processes within these systems are the mixing dynamics of dissimilar water masses and biogeochemical cycling between organic and inorganic forms. Coupled hydrodynamic-biogeochemical models can integrate these fundamental processes, enhancing our conceptual understanding of system dynamics while identifying knowledge gaps and informing management actions. Here we illustrate two applications of a hydrodynamic-biogeochemical model to better understand nutrient-bloom dynamics as well as juvenile fish dispersal in the estuarine Gippsland Lakes.
Our model consists of a hydrodynamic component simulating transport and mixing in the water column and a biogeochemical/ecological model describing processes in the water column and sediment compartments. Apart from standard processes common to these models, we also included benthic bioirrigation and salinity-dependency grazing as potentially significant yet poorly understood factors influencing nutrient cycles and phytoplankton population dynamics (highlighting two important knowledge gaps).
We used the model to explore the sensitivity of Nodularia spumigena bloom development to different physical, biological and ecological factors. We found temperature and salinity are the primary factors initialising Nodularia blooms in the lakes; phosphorus controlled the duration, size and severity; bioirrigation amplified sediment-nutrient exchanges and the formation of Nodularia blooms; and sediment phosphorus release supplied most of the phosphorus supporting Nodularia bloom development.
We combined the hydrodynamic model with field sampling to explore the relative importance dispersal and habitat quality in distributions of juvenile black bream Acanthopagrus butcheri. Preliminary results showed that dispersal of juvenile bream within the lakes was limited to areas directly influenced by river plumes; recruitment was absent in areas remote from the plumes (even those with ideal seagrass habitat). These findings highlight the utility of hydrodynamic models for identifying critical nursery habitats for fishery species and prioritising habitat-focused management actions.