Responses of benthic macroalgae to high frequency
upwelling on the Florida reef tract.
Principal Investigator: Dr. James Leichter, Scripps Institution of Oceanography

When you stand on a beach and look out to sea what is visible on the ocean’s surface reflects only the smallest fraction of what lies below. If you could take a slice of the ocean and see it in three dimensions - from the surface to say 1000 feet deep - you would be amazed at how things like temperature, dissolved nutrients, currents, and life itself, changes as you go from shallow to deep. And the changes are sometimes all mixed up and not easily explained by just looking at the slice. Physical and biological oceanographers spend much of their time trying to understand all the variable properties of the ocean and how they change with depth, over time, how they affect what lives in the ocean, and ultimately how this affects everything from the fish we eat to our climate and weather.

The current Aquarius mission is focused on studying an interesting - and important - phenomenon that occurs in the offshore waters of the Florida Keys. Over the last several years, multiple lines of evidence have led to a detailed description and understanding of how naturally-occurring pulses of cold, nutrient-rich water move from deep offshore waters onto the shallower slopes of Florida Keys coral reefs. Pulses of this cold water are associated with rapid changes in physical conditions such as current speeds and direction, where drops in temperature of 2 to 10 degrees Celsius can occur within minutes. Associated with these cold pulses are greatly elevated concentrations of dissolved nutrients. Dr. Leichter’s team is using Aquarius to investigate the interaction of these cold-water pulses (known as upwelling events, or more technically, upwelling events caused by internal tidal bores - see the animation link below) with the variable roughness of the reef surface itself. The roughness of the reef exists at different scales, produced for example by spur and groove coral formations, large coral heads and sponges growing up off the bottom, and the slope of the reef itself. The interaction of the cold water events with the topography of the reef (topography is the technical term for roughness) produces a lot of patchiness in how the cold water moves across the reef; this has consequences in terms of nutrient availability for important members of the reef community, particularly benthic macroalgae, or seaweeds. Dr. Leichter’s team will map topographic features of the reef in detail and they will sample the distribution and tissue chemistry of seaweeds in a variety of reef microhabitats (to learn how the upwelling events affect their nutrient chemistry). Using new technology - a custom designed network of oceanographic sensors including over 100 individually placed temperature sensors - the scientists will be able to monitor the upwelling events on the reef in unprecedented detail.

From previous missions and other studies, it is well known that during the summer (May through September) the temperature and density (a property of seawater affected by temperature and salinity) of the offshore water is highly stratified and a sharp thermocline (where the water temperature changes quickly from warm to colder) is found, generally between 50 and 80 m depth. It is also well known that concentrations of dissolved nutrients (nitrogen and phosphorus) are generally quite low and often undectable in near-surface waters. However, concentrations below the thermocline can be as high as 2 to 8 ?M for nitrate and up to 0.5 ?M for phosphate: these amounts are massive compared to what is typically seen on the reef itself. Thus, while nutrient concentrations on the reef tract are generally low to undetectable, a significant, and entirely natural source of nutrients lies within 1 to 3 km offshore of the reefs, just below the thermocline. While the thermocline can serve as a barrier to the vertical movement of nutrients, the horizontal or cross-shelf transport associated with the upwelling events (caused by the internal tidal bores - see animation) can periodically move high concentrations of nutrients directly onto the reef. The magnitude of these upwelling events appears to be affected by interactions among tides, the position of the Florida Current, and the depth of the offshore thermocline. Based on data from instruments Dr. Leichter deployed throughout the Florida Keys, he discovered that these upwelling events can occur at the same time across many different reefs, often separated by tens of kilometers.

Although there is considerable interest in understanding the nutrient dynamics of the Florida Keys reef tract, the highly variable nature of nutrient availability is poorly understood and the consequences of upwelling on processes that affect the structure and productivity of animals and plants that comprise the coral reefs are unknown. This project builds on the results of previous studies to identify the mechanisms of high frequency upwelling to now examine the consequences of upwelling on populations of benthic seaweeds: and it is well known that seaweeds have increased in abundance and distribution throughout the Keys, sometimes to the detriment of corals.

A press release is available that describes the results of recently published work by Dr. Leichter based on a 2001 Aquarius mission.

A PDF file of a recent publication is available based on work Dr. Leichter conducted during a 2001 Aquarius mission.

A short cartoon animation is available that shows what upwelling (internal tidal bores) looks like as it moves across in Pinnacle similar to what is found in the Gulf of Maine.

Two figures are also available that show what the results look like from the temperature array deployed on Conch Reef. Figure 1 and figure 2 show 24 hours of data collected with the BOA-II system during a test deployment at Conch Reef, May 2003. The system was deployed spanning depths from 32 to 15 m. Figure 1 shows the entire dataset from 10 arrays (100 nodes). Figure 2 shows a 16 hour section of the data from a single array (10 nodes running up the reef slope). A large cooling event is evident during this time, with the extent of cooling increasing with depth, and with fine scale variability among node locations. Horizontal spacing is 15 m between nodes, data are sampled at 10 s intervals.