We’re moving along now, with some gear problems have been sorted out and we’re back in the swing of things. We’re heading to our northern station shortly, as soon as we squeeze in another MOCNESS tow. The retermination of the conducting cable went well, and the first tow with the MOCNESS was a smashing success. Pictures to follow, and I’ll be posting Twitpics on @planktoneer
As far as the science goes, I’ve been trying to figure out what to say, but it’s such a different year and different from our expectations that it’s hard to categorize other than the comments I’ve already made. Since arriving at our southern station though, salinity has been climbing. It’s not likely due to evaporation, though it’s awfully hot out on the steel deck in direct sun. It’s more likely due to the spring freshet finally abating. All of the snow received in the northeast, particularly Pennsylvania has been causing high river flows into the Chesapeake watershed, dropping surface salinity well below the long term average. This suggests that there may be a lot of hypoxia this year, or perhaps just earlier hypoxia depending on what happens later in the summer. All of that extra flow into the bay brings nutrients and enhances stratification, which means that there is a bigger difference between surface and bottom salinity this year than in an average year. That salinity difference causes something of a barrier to oxygen and nutrient exchange. So, the algae in the surface will use up all those nutrients coming in with the freshwater, grow really fast and either be eaten or die. Either way, the remains (either dead cells or “processed” by the animals eating the algae) sink to the bottom and get decomposed by bacteria. That decomposition process uses up oxygen, and voila – hypoxia. Now, the only way to get oxygen into the water is either by photosynthesis of algae or exposure to air. Deep water has little light, and even less on a year like this because of the high algae production and detritus in the freshwater. Plus, the enhanced stratification keeps the mixing between deep and surface water to a minimum.
Of course it could never be that easy. There is some new work coming out from some folks at Old Dominion University in Virginia suggesting that wind direction, more than magnitude even, and help ventilate the deep water of the Chesapeake. What happens is basically that the wind can blow some of the surface water away, causing something of a hole, which gets filled in by deeper water. This allows more contact between the deep water and the oxygenated surface water, or in some cases directly with the atmosphere. Couple that to dropping nutrient inputs throughout the summer as the spring freshet subsides, and suddenly hypoxia is not nearly as strong as it could have been if it were just up to nutrients alone.
These are some of the ideas that drive a program like ours, particularly in regards to sampling repeatedly throughout the year to track monthly or seasonal changes. The differences between years highlight the need to make measurements over even longer time intervals too, because there is a lot of variability between years that must be accounted for. If this was our first year of the project, we would likely have very different expectations for what was going to happen next year than we did at the end of last year.
Science is like puzzle, and it keeps us on our toes. The most fun part for me is putting all the pieces to gather to see how they fit, and then trying to find the stories in the data. The data always tells a story, and often we learn the most when it’s a very different story from the one we were expecting.
Now, it’s time to get more data.