Aquarists from the New England Aquarium freqently travel to Belize as part of a long-term research program by Aquarium scientist Randi Rotjan, PhD, to monitor coral health near Carrie Bow Cay. Today's post by aquarist Joe Masi is part of a series about these research efforts. 

Sea conditions continue to be near perfect around Carrie bow cay, light winds and hardly any swell. This makes reef survey work very easy for the group. Wind and waves can make boating difficult and, while diving in shallower depths, tough stay one place to measure coral. But what about sea conditions for the animals we are surveying?

Measuring the turbidity of the water

Meteorological and oceanographic conditions have been recorded on Carrie Bow Cay since 1993 as part of the Caribbean Coastal Marine Productivity Program (CARICOMP), which focuses on studying the interactions between land and sea. Daily precipitation, air temperature, wind direction, sea temperature, and salinity are a few of the parameters recorded. This unique data set reveals seasonal changes, short term events (hurricanes and storms) and long term trends.

A Secchi disk is used to measure water transparency (turbidity). The disk is lowered into the water until it can no longer been seen. This depth is recorded as a measurement of the transparency of water.

One of the more interesting conditions recorded is water transparency. Water transparency, also known as turbidity, is a measurement of how much suspended material is floating in the water column. Many influences negatively affect water transparency, including wind, rain and land-based sources, such as water runoff from deforestation, increasing farming activities and fertilizers, coastal development and associated loss of mangrove habitats and harbor dredging. Unfortunately, many of these manmade influences are occurring today in Belize and neighboring Honduras.

Water transparency around Carrie Bow Cay is very seasonal. From March to June, northeasterly airflow and less rain result in higher water quality. October to June water quality is reduced because of higher rainfall amounts and increased west winds pushing coastal water out over the reef (Koltes and Opishinski). It is this wind and rain that transports all those negative impacts discussed earlier out onto the reef. Seasonal changes are to be expected and natural, but more troubling is the long term data that proves a dramatic loss in water clarity since monitoring began in 1993. The mean annual Secchi disk distance declined from 23.8m in 1993 to 15.6m in 2008 in the area over our surveys sites (Koltes and Opishinki). That is roughly 0.5 m/year and over 25 feet in 15 years.      

Here's the science!

So, how does decreased clarity affect the corals and fish? Most corals and their symbionts require light to survive, as clarity reduces so does the available light for these organisms to grow and survive. Pollution and toxins from runoff can stress corals and make them more susceptible to disease. Sedimentation can smother corals and impede larval settlement during spawning. Water with a high nutrient load from farming promotes macro algae growth, which competes for space with coral, and algae blooms, further decreasing water clarity. Studies have shown reefs can survive in turbid waters but only at shallow depths and lower diversity of species.  

I cannot say definitively that coral diversity and numbers are declining around Carrie Bow Cay and by what means. Years down the road these surveys could tell. Based on the data discussed, I assume water transparency is a big problem.

Stay tuned for more from Joe. And catch up on previous trips to Belize—lots more amazing pictures!

• Researchers most recently visited this past spring
• See the beauty of hermit crabs and ride out a tropical storm during their 2013 trip
• Learn more about threats to corals, plus signs of a late-night visitor to Carrie Bow Cay, in 2012
• See what other researchers are up to at the research station in 2011
• And read the exciting post where the marine protected area was announced in 2010