by Kate Hanson
Abstract:
This study will characterize patterns in circulation on fore reef and lagoon habitats. The resulting information will allow investigations of nutrient supply (both natural and anthropogenic) and the transport of pollutants, sediment, and marine organisms both into and out of near shore reef habitats. Research of this nature is of immediate value to the design of marine reserves and management of anthropogenic impacts. More broadly, it furthers our understanding of how the growth, reproduction, and survival of corals, fish, and other reef organisms are affected by changing environmental conditions.
Introduction:
This study examines circulation patterns in reef and lagoon habitats surrounding Cook s Bay, located on the north-eastern coast of the island of Moorea (Fig. 1). Specifically, this project will 1) determine the horizontal and vertical structure of the circulation; 2) identify the dominant forcing mechanisms for circulation (e.g. wind, tides, waves); 3) determine the effect of circulation for coral and other reef populations. This study will also examine the effect of stratification on circulation and how such stratification changes with season according to fluctuations in freshwater input and solar radiation.
Cook s Bay is an ideal site for studies of circulation both because of the hydrodynamic complexity inherent to reef habitats and because of the wealth of local information available to link circulation patterns with larger ecosystem processes. Because of its proximity to the UC Berkeley Gump Research Station, Cook s Bay and the surrounding reef habitat have been the focus of more than fifteen years of scientific investigation, including research on spatial and temporal distributions of biotic communities, evidence and effects of increased sedimentation, and settlement and recruitment of a variety of fish and invertebrate species. Furthermore, Cook s Bay has been significantly impacted by anthropogenic sedimentation resulting from both terrestrial runoff and boat activity. Cook s Bay is frequently visited by tourists and impacted by the associated traffic, including the weekly docking of cruise ships. There is a severe gradient in degradation and reduced water and habitat quality from the mouth of the bay to the back of the bay. While many of these differences are related to anthropogenic impacts, studies of planktonic and benthic communities have demonstrated natural gradients in community composition unrelated to spatial gradients in anthropogenic influence.
Figure 1: The Island of Moorea. Cook's Bay and general research area indicated in boxed region. Map and text taken from Dufour et al. (1996)
Fig.1. Moorea Island (17°30'S,149°50'W), Society Archipelago, French Polynesia. The five sampling sites around the island are indicated by triangles. The bold lines around the island represent the reef crest. The lagoon is the area between the reef crest and the island.
Methods:
Oceanographic equipment including current meters, temperature sensors, light meters, and conductivity meters have been deployed on the fore reef slope and lagoon in and near Cook s Bay, Moorea. Short term (~13 hour) measurements of current and water properties will be made from a small boat by lowering instruments into the water. Long term measurements (>1 day) will be taken by anchored instruments and the data recorded internally. Oceanographic sampling will be conducted from small boats as well as instruments deployed on SCUBA.
During this period of time, various oceanographic sampling devices were assembled and deployed. Previously deployed instruments were recovered, calibrated, and internally recorded data was retrieved. In addition, sites for future deployments were scouted.
This field week included the recovery as well as new deployment of numerous acoustic current profiling devices (ADCPs and ADPs) at five locations spanning the fore reef slope, back reef and lagoon areas, and the channel marking the entrance to Cook s Bay (Figure 2). Individual thermistors and CTDs were recovered and re-deployed from these areas and buoyed thermistor chains were added to lend the perspective of vertical gradients in temperature and their change through time.
Figure 2: (Left) Tripod-mounted upward-looking ADCP deployed ~25 m depth on the fore reef northwest of Cook s Bay. (Right) ADP (foreground) and CTD package deployed in the patch reef habitat characteristic of lagoon areas surrounding Cook s Bay (Photo credits: J Hench, 2004).
Implications & Future Research:
Identification and characterization of circulation patterns in Cook s Bay and the surrounding reef is a necessary component of any marine conservation planning or management in the area. Knowledge of physical transport processes in the area will not only aid in mitigation of anthropogenic pollution and sedimentation, but will add a significant physical dimension to local studies of reef community dynamics through space and time. This dataset will set the stage for long-term studies of physical-biological interactions in the area, and will contribute to a new NSF-funded Long Term Ecological Research program currently being built in Moorea.
Personally, this field work has and will continue to contribute to my doctoral studies. . I added to my scientific diving knowledge by learning how to handle, calibrate, and deploy oceanographic equipment on SCUBA. The dives I made during this process are also a significant advancement toward my 100 ft. depth certification. Furthermore, I anticipate utilizing this type of oceanographic instrumentation (ADCPs, CTDs, thermistors, etc.) and the associated data sets as a major part of my thesis research. Thus both the working experience and data collected during this field work will be of integral importance to my future doctoral research.
References:
Dufour, V, Riclet, E, and Lo-Yat, A (1996) Colonization of reef fishes at Moorea Island, French Polynesia: Temporal and spatial variation of the larval flux. Marine and Freshwater Research 47:413-422.
