Fine scale surface currents in Cid Harbour, Whitsunday Islands, Great Barrier Reef

Aerial photographs, releases of numbered paper drogues and fluorescein dye were used to study fine-scale surface currents in Cid Harbour, Whitsunday Islands, during August 1975.Initially, existing aerial photographs were used to identify gyres formed by the tidal current behind rocky promontories or small islands, eddies, current directions and mixing patterns within the harbour (shear zones, frontal systems, convergences and divergences). Additional colour aerial photographs were taken at specified intervals from a height of 800m and the apparent rotation of the gyres in the photographs was compared to the rotational speed of the current measured from the position of drogues released from small boats within the gyres or eddy currents.Sets of 10-20 drogues were released from different locations within the harbour and at various times throughout the tidal cycle, to provide quantitative information of rates of flow and direction of water flow. Information on tidal cycles used to determine the timing of deployment of drogues was taken from predicted tides for the outer harbour for the city of Mackay. Each drogue in a set was deployed individually, with a specified distance between the release of each drogue. Each set of drogues was released over a period of 3-6 minutes. Corrections were later made to approximate simultaneous release. Observations of wind speed and direction were made at half hourly intervals during the release of drogues. Most drogues were released when the wind speed was Fluorescein dye was released from small boats and the dispersion of the dye photographed from an aircraft, to obtain finer scale measurements of current distribution.
This study was initiated to obtain descriptive data on the fine-scale distribution of surface water currents in Cid Harbour, to complement studies of patterns in the distribution of zooplankton in the same area.