Mass Mortality – A Natural Part of Life in the Oceans.

The ocean and its living inhabitants form a complex system which is really very poorly understood.

Recently, we have had large collections of dead and dying scallops, pen shells, sea urchins and parchment worms. One would immediately assume that some form of water quality issue is responsible, and it probably is – just not the types of water-quality issues to which we have become accustomed.

It appears that perhaps this die-off is best attributed to a sudden influx of cold water from out in the deeper regions of the Gulf. Prior to a couple of weeks ago, the water temperature was in the lower 70s (F), then we experienced a large upwelling event that brought cold water from deeper regions of the Gulf resulting in water temperatures of 63 ° F around our island.
The density of water is inversely related to temperature (at least above freezing.) Water that is colder is more dense. The very deep portions of most ocean basins hold very cold water. For some reason, some event likely happened that caused some of this cold water to upwell – that is travel towards the surface. The triggering mechanism causing the upwelling was probably a series of strong winds. These winds push surface water away and pull up bottom water to replace it.

This cold water then rushes past many of our bottom-dwelling invertebrates – known in biological circles as benthic invertebrates. These creatures do not readily thrive in the face of rapid temperature changes and become ‘cold-shocked’ or at times just die from the rapid temperature change. Once weakened, they are unable to maintain their hold on the bottom and become tossed and carried about by the currents. Many of which are then washed ashore on our beaches.

Some parts of this explanation are based on conjecture, but what we do know is that we experienced an upwelling event offshore (we can track surface ocean temperatures by satellite) and we had a rapid decrease in ocean water temperature around Sanibel. We also know that many of the invertebrates we have seen recently on our beaches are vulnerable to rapid temperature change. With this information it seems plausible that our recent die-offs are a result of temperature-shock; not red tide nor pollution, but just another interesting wrinkle of the dynamics of the ocean and its inhabitants.

There is an old adage in medicine that says when you hear hoof-beats don’t assume zebras (in North America) – sometimes what appears to be a result of some horrific pollution event is just the natural oscillation of life and death in the sea. Something we don’t know very much about.
We need to systematically monitor the life (and death) that washes onto our beaches. Along with other conservation organizations and the State of Florida, we are creating a network of beach-combing volunteers to do just that. Together we can truly contribute to our understanding of the dynamics of our oceans; if you are interested in joining this effort email me at bruce@sanibelseaschool.org

Perhaps together, we will get better at distinguishing the horses from the zebras.