Climate change has become omnipresent. There are concerns of sea level rise, changing precipitation patterns, global warming, ocean acidification, species extinctions, intensity of fires and hurricanes, you've heard them all. What is the scientist's approach to study the impact of climate change? Well, there are many avenues, and as a graduate student with a high degree of interest in the sensitivity of marine animals to climate change, everyday I am refining my approach in order to contribute meaningful and sound science of how marine animals interact with their environment and how they will be impacted by anthropogenically-driven climate change.
In order to investigate the effect climate change has on marine animals, we must first have an in-depth understanding of their life history and ecology. Let's take the sand dollar as an example.
Divers along the West Coast can tell you they form dense beds along sandy bottoms. They also position themselves upright - by inundating their bottom third into the sand for support - to efficiently feed on small particles floating in the water. And of course, we can all identify this animal from the pure symmetry and beauty of its skeleton.
The life history of a sand dollar is quite complex yet fascinates the naturalist in us all. First of all, how do the sand dollar populations persist from generation to generation? In the simplest terms, adults need to survive and reproduce; the resulting individuals must grow, mature, and reproduce themselves. This sounds easy enough.
It's not. To reproduce, adults broodcast spawn - meaning males and females release sperm and egg into the water column to be fertilized, which is risky. Nearshore currents are always moving which dilutes the sperm and egg. So how will sperm and egg ever find each other? Well, this is one reason why adults aggregate, coordinate spawning events, and increase the likelihood of fertilization. In addition, the females don't just produce a few eggs, they produce hundreds of thousands, and the males don't just produce hundreds of thousands of sperm, they produce billions.
After fertilization, what's next? The resulting embryo disperses with ocean currents and develops into a form which shares no resemblance to the adult. The larval form begins to feed within days and disperses for weeks to months. This dispersal phase is very important to sand dollar populations along the west coast. Larvae produced in one location can drift with the currents, grow up, and settle into a population down the road. The settling phase is a stressful time for the sand dollar larvae; the larvae find (but not always) a suitable habitat, and initiate metamorphosis. During metamorphosis, feeding stops and morphology reorganizes to resemble the iconic adult form. A successfully metamorphed juvenile grows, matures, and must successfully spawn for populations to persist.
This is the general life history of many marine invertebrates: sea urchins, sea cucumbers, mussels, sea stars. Now that we are familiar with the basic life history patterns of coastal invertebrates, we can begin to think about what restricts their survival, and what happens when their environment starts to change.
Marine Organisms and Climate Change
The sand dollar's environment has been altered, is being altered and will be further altered by climate change. For example, sea surface warming affects adult populations, and also impacts larvae and juveniles. Furthermore, it's not just temperature, but changing ocean chemistry as well. Up to 40% of the carbon dioxide humans produce is absorbed by the ocean. The forms of carbon dioxide in seawater - carbonate, bicarbonate and carbonic acid - change proportionally as a function of the amount of carbon dioxide added. This in effect alters the pH of seawater, and the result is ocean acidification. So why is ocean acidification a concern for our sand dollar? Because its skeleton is a form of calcium carbonate, and the stability of calcium carbonate depends partially on the pH of the seawater. Thus far, laboratory experiments on calcifying species suggest that calcification rates decrease under ocean acidification scenarios.
The difficult part of science is quantifying whether a decrease in calcification of an individual will have a negative impact on the population as a whole. Furthermore, not only are we concerned about acidification for these populations, but also sea surface warming, habitat change, increased stratification, altered climatic patterns which could result in more or less rain and runoff. It seems as if this exhaustive list is discouraging, but it shouldn't be. There is a great deal of effort to answer these questions. And while there will always be uncertainty, we are learning a great deal about our coastal habitat, and how the animals within it persist from generation to generation.
