Mission 3 — The role of Hydrodynamics in Determining Nutrient Fluxes to Conch Reef.

Mission 3 — The role of Hydrodynamics in Determining Nutrient Fluxes to Conch Reef.

Principal Investigator: Dr. Stephen Monismith

Training: July 5 — 8, 11
Mission: July 12 — 21

Central to the study of coral reef ecosystem processes is the study of hydrodynamics. Large–scale flows (on the order of a kilometer or more), such as tidal fluctuations, local currents, and internal waves, control the local environmental conditions around a coral reef through the bulk transport of nutrients, planktonic prey, and larvae. The ability to predict large–scale horizontal transport patterns is essential to modeling the connectivity of coral populations or designing and managing marine reserves and fisheries. The interaction of large–scale flows with the rough bottom topography of coral reefs creates small–scale flows that govern the vertical transport of material from the water column to the benthos. The roughness of benthic reef communities causes currents and waves to dissipate energy on the reef at rates which far exceed ocean values of energy dissipation. This physical characteristic of corals allows them to maintain a high level of productivity in very low–nutrient waters.

It has long been accepted that wave action has a dramatic effect on the structure of bottom boundary layer dynamics. Most of the research to date addressing wavedynamics on coral reefs has focused on boundary layers, vertical mixing, and masstransport due to surface–gravity wave transformation and forcing over and along fringing reefs. Reports of internal waves on coral reefs are relatively few. There has been even less work addressing the effects of breaking internal waves or internal bores on coral reef hydrodynamics. In a series of pioneering studies, Leichter et al. (1996, 1999, 2003) documented in detail the presence of persistent, high–frequency internal bores generated by breaking internal waves on the Florida Keys reef tract. Leichter et al. (1998) found the internal bores to be an important mechanism for the cross–shelf transport of phytoplankton and larvae to the reef. While the nature and importance of high–frequency physical variability to coral reef ecosystems are as of yet poorly understood, it is reasonable to believe that the intermittent delivery of cold water, rich in nutrients, phytoplankton, and larvae, to organisms which usually flourish in oligotrophic environments with limited seasonal temperature variability will significantly effect the local temperature regimes, nutrient uptake, and recruitment and feeding dynamics.

Motivated by the apparent alteration of large–scale flows and boundary layer dynamics by the internal wave field and the importance of these flows to coral reef ecology, this project addresses four major objectives:

Characterize the large–scale flow field on and around Conch Reef
Measure the nutrient concentrations, activity ratios of radium, temperature, salinity, and chlorophyll signals for each of the water masses in contact with the Conch Reef
Understand how the breaking of internal waves changes the bottom boundary layer dynamics and vertical mixing on the reef
Assess the conditions for the field site for a future “engineering control volume” experimental setup.

To address these objectives we conducted two day–boat operations from the NURC Key Largo facility, the first in September 2003, and the second in collaboration with Jim Leichter during his June 2004 Aquarius mission.

During the September 2003 field operation an Acoustic Doppler Current Profiler (ADCP), a Conductivity, Temperature, and Depth Profiler (CTD), and several thermistors (Seabird–39) were deployed on the reef for nine days to collect data on the large–scale flow dynamics on the reef during internal wave events. Two 4.5 km cross–shore transects were completed by a REMUS Autonomous Underwater Vehicle (AUV), 2 collecting large–scale hydrographic data and side–scan sonar images that will be used to characterize the bottom roughness on the reef. Water samples and CTD measurements were taken in depth profiles in a 6 km cross–shore transect from the back reef to waters offshore of the reef. Several dives were conducted to survey the reef areas near Aquarius to search for a suitable place to set up an experiment site for a future Aquarius mission.

During the June 2004 field operation three ADCPs, 15 Seabird–39 thermistors, and three Nortek Vector Acoustic Doppler Velocimeters (ADV) were deployed on the reef for two weeks during a period of strong internal wave events. These instruments collected data on both large–scale flow dynamics and small–scale bottom boundary layer dynamics on the reef. Four offshore CTD casting transects and water sampling profiles were completed to characterize the offshore stratification. Several time–series profiles were taken with a microstructure profiler (SCAMP) from which dissipation and vertical mixing on the reef can be inferred. Divers conducted coral reef rugosity measurements to quantify the hydrodynamic roughness of the reef and to compare to the Images taken in September 2003 by the REMUS AUV.