Researchers from The University of Western Australia Oceans Institute have been studying how giant tides in the west Kimberley (up to 12 metres range) flush reefs with water rich in nutrients and phytoplankton, which are food sources for seagrass, algae, coral, and other reef organisms.
In a journal article recently published in Limnology and Oceanography, the scientists, working on the Western Australian Marine Sience Institution’s (WAMSI) Kimberley Marine Research Program, showed how semidiurnal and spring-neap tidal cycles helped control nutrient availability on reefs.
Professor Ryan Lowe, Dr Renee Gruber, and Dr Jim Falter worked on Tallon Island, a fringing reef in the Sunday Island group, with assistance from the Bardi Jawi Rangers and Kimberley Marine Research Station staff.
The researchers placed instruments on the reef to measure flow speeds and chlorophyll concentrations in waters flowing over the reef. Chlorophyll is a pigment present in green plants and is used to estimate how much phytoplankton (a food source for coral) is present.
The researchers also built a four metre high scaffold on a sandy part of the reef to hold an automatic water sampler that collected water samples throughout the tidal cycle. This study occurred over three weeks in order to measure how the conditions varied over a complete spring-neap tidal cycle.
|Platform and water sampler on Tallon Reef during high tide (Renee Gruber)|
Many Kimberley reefs sit close to mean sea level and become “cut off” from surrounding ocean waters when the tide falls below the level of the reef crest. The researchers found that during these periods, almost all chlorophyll is grazed from the water column and reef organisms must wait for the next flood tide to feed again.
By measuring the chlorophyll entering and exiting the reef each tidal cycle, the researchers determined how much feeding occurred over the entire reef platform and estimated that phytoplankton provided ~50% of the nitrogen used by the reef community.
Although approximately 30% of reefs worldwide experience water motion driven predominantly by tides, almost all scientific studies of reefs to date have focused on locations where the flows responsible for ocean-reef nutrient exchange are driven mainly by currents generated by breaking waves.
|Giant clams are an abundant filter-feeding organism on Tallon Reef (Renee Gruber)|
“Our results are an important first step in understanding how tides can affect the productivity and growth of reef communities,” Dr Renee Gruber said. “The physics of water motion control many aspects of an organism’s life cycle, and we must first understand the physics before we can predict how future challenges such as sea level rise and ocean warming will affect tidally-driven reefs.”
This study was also among the first to publish water quality data for the coastal Kimberley region, which is a first step in helping managers set baselines and interpret future changes in environmental condition in the broader region.
Gruber R, Lowe R, Falter J (2018) Benthic uptake of phytoplankton and ocean-reef exchange of particulate nutrients on a tide-dominated reef. Limnology and Oceanography doi: 10.1002/lno.10790
Project Page: www.wamsi.org.au/benthic-community-production
The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley.