27 June 2016
The Kimberley is known to have a unique geography and support an amazing amount of biodiversity, but until know we have not really understood what drives the base of the food web - what we call productivity. Is it nutrients entering coastal regions from the ocean or is loading of material from land via the complex estuary systems suppling the essential nutrients required to drive production?
Given the high-tidal range and very unique and complex coastal morphometry, we also want to understand how nutrients and the key players in the microbial food web are impacted by seasonal changes in ocean currents and mixing. Overall, we have therefore set out to understand the hydrodynamic and biogeochemical controls on productivity, and then to use this information to consider how future changes will impact these systems.
Simulation results showing the effect of inflows and sediment resuspension on turbidity (“Suspended Solids”) within the coastal embayments of Collier Bay and Walcott Inlett
Both the regional-scale and local-scale dynamics within a given reach of coast will ultimately determine how it functions. We have identified at both scales the importance of the large tidal range in shaping nutrient availability.
Somewhat paradoxically, at the regional scale tidal and open ocean dynamics encourages upwelling of nutrients from the deep ocean and into the coastal regions, but at the embayment scale the large tides and induced strong currents (up to 3 m/s) are responsible for flushing terrestrial derived nutrients away from the coast. Nutrient supply is therefore highly dynamic in time and space, leading to complex patterns in primary productivity across the Kimberley shelf and within the coastal archipelagos.
In the dry season conditions, water masses can be retained in embayments for time-scales varying with exact location but lying in the range between just a few days up five months. In the wet season conditions, there is enhanced flushing and these timescales are approximately halved.
Particle tracking animation demonstrating the complex currents and flow paths experienced within Collier Bay
Whilst the delivery of nutrients is an important driver of primary productivity, within Kimberley estuaries and embayments, this is complicated by highly turbid water, limiting light available for photosynthesis, particularly near the coast.
Model simulation results showing the variation in physical and biogeochemical parameters across Collier Bay (from within Walcott Inlet to the outer shelf area of the Kimberley) during the March 2014 wet season. The parameters are Salinity (psu), Suspended Solids (mg/L), Nitrate and Dissolved Organic Nitrogen (mmol/m3), and two types of phytoplankton (mol/m3).
Despite the low light levels, the intense tidal flows sustain high rates of vertical mixing and the phytoplankton community has uniquely adapted to these niche conditions, being able to rapidly photosynthesise in the thin layer near the surface for short periods, differentiating them from their open-ocean counter-parts that are most productive deeper within the water column.
Overall, the rates of primary production across the region were as high as other tropical waters in Australia, and interestingly the rates of secondary production - that is grazing of phytoplankton by zooplankton - were reported to be significantly higher than the Great Barrier Reef, providing a rich food supply to local food-webs.
Model simulation results showing the variation across Collier Bay (from Walcott Inlet to the outer shelf area of the Kimberley). The key parameters (TCHLA: total chlorophyll-a; GPP: gross primary productivity; fI: light limitation; fNIT: nutrient limitation) for assessing productivity demonstrate the transition between nutrient and light limited environments.
These findings have allowed the development of several high-resolution model tools able to compute the changes in oceanographic conditions, nutrient cycling and primary production that can be used to help understand and manage the system into the future.
2.2.1 Oceanographic dynamics
2.2.2 Biogeochemical processes
2.2.6 Land-ocean linkages
The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government's Kimberley Science and Conservation Strategy co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley./Nike_7_1