Ocean Observations - Coral Reef Research Foundation
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Ocean Observations

To understand the biology and ecology of the marine environment, we need to understand the physical processes and their natural variations that influence the different marine habitats.

Physical oceanographic instruments allow measurements of important processes affecting the marine environment.  Some of the instruments used in Palau are:

  • Pressure Sensing: Sea level, tides and waves
  • Acoustic Doppler Current Profilers (ADCP): measure currents throughout the water column.
  • Satellite tracked sea gliders: Buoyancy driven vehicles that do dives to measure physical parameters to 1000 m the report data in near real time. Data include Temperature, salinity, chlorophyll fluorescence: Water column structure with depth.
  • Multi-beam sonar: measure water depth and shape of the seafloor (bathymetry)

CRRF participates in this work throughout Palau in collaboration with our colleagues from Scripps Institution of Oceanography, the University of Delaware, University of Hawaii and the Office of Naval Research.

Sea Level, Tides and Waves

While the tides may seem a simple parameter to understand (they go up and they go down mostly due to gravitational effects of the moon and sun), they are affected by many external factors.  Hence tide tables are nothing more than predictions, based on past measurements, and the actual sea level data making up the tide can differ.

Palau has an excellent tide gauge, run by the University of Hawaii Sea Level Center (UHSLC), located on Malakal Island which has provided quality data since 1969.  The tidal range in Palau between high and low tide, on a spring tide, can be as much as 2 m (78 inches).

There is local belief that the tide state can vary from north to south within Palau by as much as an hour.  We asked this question early on by deploying our own array of a dozen pressure loggers (“tide gauges”) in 2010. Data showed little actual variation in tide geographically within Palau.  But it also showed that wind and waves could produce some differences in the timing of tides north to south. At present, for our own purposes CRRF now has two permanent stations:  1) CRRF dock and 2) Malakal Harbor to compliment the Malakal tide station readings.  CRRF also maintains a series of loggers in various marine lakes, as the tides in the lakes are damped and delayed.

An important aspect of climate change is the mean sea level (MSL) variation.  In Palau, between 1969 and 2016 mean sea level has varied, with the most dramatic changes associated with El Niño and La Niña events.  Overall the mean sea level in this 46 year period has increased about 2.3 mm per year. In the short term, one or two years, the results can be different.  In the past decade Palau’s sea level was reported to be rising 9 mm/yr, based on satellite sea surface height data from 1992 to 2010 (red regression line in Figure).  This was actually true for that time period, but expanding the record both before and after indicates the overall rate falls back to less than 3 mm a year and is consistent with worldwide sea level rise rates.

A great example of short term mean sea level change was during 2015-2016.  Mean sea level data for Palau in 2015 clearly showed the strong El Niño, where MSL decreased considerably throughout the year, with an average decrease of 90 mm/yr. (red line on Figure). This trend, however, was short lived.  As the El Niño waned towards ENSO neutral, in March 2016 the MSL rose rapidly to 40-60 cm higher than in 2015!  Between 1 March and 15 May 2016, MSL rose 560 mm (56 cm), an average of 7.5 mm PER DAY! The overall trend for the two years was about 205 mm (20.5 cm) upward per year, more than a half mm per day.

Currents – Oceanic and Local

Palau is situated in an area of active oceanic currents in the western Pacific. The North Equatorial Current (NEC) is usually found running east to west just north of Palau, but often impacts the eastern side of the main Palau group.  The North Equatorial Counter Current (NECC) moves the opposite direction, south of the main Palau island group. These oceanic currents hit the face of Palau’s outer reef, as the reefs are very steep and drop off to oceanic depths from shallow water quickly.

These currents are important as they are the connection of Palau with the rest of the ocean.  They transport water masses with larvae of marine organisms to or from Palau and are critical in concerns about fishing, search and rescue, pollution and marine debris. Much of the information on ocean currents is based on satellite remote observations, such as sea surface height or tracks of drifters in the open ocean. Islands and reefs interrupt the unobstructed flow of the ocean currents, producing patterns of circulation which are hard to model and predict. Data gathered locally through direct instrument measurements of critical in understanding the relationship of island/reefs with the surrounding ocean, information that is important for navigation, fishing, marine resource management and search and rescue efforts. Such models are being constantly refined, based on new data such as that gathered in Palau.

Simplistic representations of these ocean currents show them as vector arrows running in fairly regular paths, with an area of eddies and other interactions between them.  Like most things in nature it is more complicated than that, with each of the major currents (NEC and NECC) being masses of eddies swirling along as the overall water mass moves east or west.  When the westward moving NEC hits the east side of Palau, it can divide into two branches, one going south to the area of Peleliu and Angaur while the other goes north to the area of the northern reef complex (Kossol and others), Kayangel and Velasco Reef.  Here the currents are squeezed between the islands and reefs becoming jet-like with accelerated current speeds and spinning off vortices at the edges of islands and reefs that migrate out to sea with the general current. The water passing around Palau also curves back in on the western side of the main group forming a series of eddies there which circulate far offshore of the western reefs or move into the area of the reefs themselves.

For the past several years CRRF and our collaborators have maintained a network of five Acoustic Doppler Current Profilers (ADCPs) on the outer reefs of Palau to examine how currents along the reef are structured.  The ADCPs measure currents at multiple depths above them and are a basic tool of physical oceanography today.

Another important tool for studying currents are ocean ‘gliders’ that gather current speed & direction, and physical information, such as temperature and salinity, over depth.  Gliders use buoyancy to make them go up and down, with wings and fins to control their track through the water.  They can relay this information to a shore base after dives to several hundred meters, and stay at sea for several months.  Gliders are largely autonomous except for communicating via satellite links with controllers in shore stations.

Dan Rudnick at SIO has been using Spray gliders in and around Palau since 2009, looking at first the transport across the North Equatorial and Mindanao Currents. More recently two gliders have been used to simultaneously profile the eastern and western side of the northern reef tract up to Velasco Reef to observe eddying in these areas. Spray information, including tracks, profiles of currents and values recorded on dives are available here.

More recently the Coastal Oceanographic Research and Development Center (CORDC) of Scripps has set up three station High Frequency Radar surface-current measuring network for the east side of Palau. Stations are located in Angaur, Melekeok and Kayangel with the system providing a picture of surface currents every hour. They provide overall estimates of water transport around Palau and near real time current speed and direction information, useful for Search and Rescue efforts. Maps of current patterns can be seen here.

Bathymetry

Bathymetry is the measurement of water depth.  Bathymetric information has many uses, including creating charts for maritime use to modeling currents influenced by underwater topography.  The most recent bathymetric surveys around Palau have  produced highly detailed images showing the structure of underwater canyons and other features, including the Palau trench.  The first bathymetric surveys of Palau were done by the Japanese prior to WWII and printed as marine charts used for navigation. GPS positioning this allowed a new level of accuracy for plotting depths on charts, as well as accurately positioning islands. It was found, for example, that some charts for Palau in use at the time had errors up to about one half nautical mile.  These errors were corrected in the US DMA charts issued afterwards.

CRRF started doing original bathymetric surveys in 2004 with specific areas of special interest targeted. Such surveys were done using “single point” sonar in which the latitude, longitude and depth are recorded while the boat makes a track through the survey area. Detailed surveys were done of the inner Rock Island areas of Palau, taking many hours and days to adequately cover these complicated areas. The data are then plotted in a contour-plotting program  producing a chart of an area.  An early example shows the bathymetry of Nikko Bay, presented as shaded depth contours with an aerial photo image of the islands.  Depth data can also be shown as white contour lines against the unaltered background image from the aerial photograph.

In 2011 Scripps Institution of Oceanography (SIO) Coastal Observing Research & Development Center (CORDC) and the R/V Roger Revelle (SIO) with CRRF started doing large scale multi-beam sonar surveys around Palau. The R/V Revelle has a full ocean depth multi-beam sonar that can image the bottom in any area of the Palau EEZ.  It is not effective at depths shallower than 200 m. In 2013 the ship did two circuits around the main Palau group to image the entire outer slope of the islands.  This allowed the preparation of highly detailed images showing the structure of underwater canyons and other features, including the Palau trench. After the initial survey circuits around the main group, the R/V Revelle continued on to each of the SW islands of Palau (Sonsorol, Fana, Pulo Anna, Merir, Tobi and Helen Reef) and performed multibeam sonar surveys around them. All these have steep outer slopes dropping quickly to depths of many thousands of meters.

Also mapped was the “Short Drop Off” basin on the central east side of Palau, going down to depths of nearly 5,000 m.  The bathymetry is shown against a satellite image of the shallow reefs and islands outside the survey. The submarine canyons and alluvial plains seen were not known previously for Palau.

As reef scientists we are also very interested in the shallow water bathymetry and less powerful multi-beam sonars, compared to the full ocean depth system on the Revelle, are used to image areas along the reef. Increasing depth from the multi-beam survey are typically shown as different colors, such as the Wonder Channel survey. The survey is shown against a vertical aerial image of the area showing islands and areas outside the survey as they actually appear.  This has allowed us to image many of the outer reef areas popular for diving, as well as special interest areas within the lagoon. The fine detail of such shallow multi-beam sonar images also allows scientists to see the roughness (rugosity) of the outer reef walls and slopes, factors that affect the currents coursing along them. Such surveys are important in understanding ocean currents along the reef, how the reefs and islands affect ocean circulation, the, geological history of the reef, and how organisms relate to and use topographic features for events such as spawning.

Multi-beam bathymetric data can also be presented as an oblique view with colors indicating depths, and showing the relief and topography of the study area.  The view of “Croc Head”, a promontory on the western barrier reef of Palau, shows vertical escarpments, reentrants and the generalize slope of the reef to depth of nearly 200 m.

A detailed perspective view of an area of Short Drop Off indicates a vertical escarpment from about 90 to 120 m depth.  This was verified by both submersible and mixed-gas diving, and is a spectacular escarpment.  A geologic features at 120 m, seen in the multi-beam images is believed to represent the low stand of sea level during the last glacial maximum.  The reef structure above this depth was actually exposed above sea level and eroded away by the elements to an extent. When sea level started rising again, about 20,000 years ago, the previously exposed reef was now submerged and reefs started growing on the limestone rock, but there was a discontinuity between the earlier “Pleistocene” reef rock and the more recent that is reflected in the horizontal feature.