Phytoplankton and the River
Patterns in algal biomass in Modoc, Mellwood, and Quapaw Chute
by Dr. Clifford Ochs (U. Miss Dept. of Biology)
and Nok Pongruktham (graduate student U. Miss)
The algae that occur in the river and adjacent lakes are called phytoplankton. They are microscopic algae, usually just one or a few cells per individual, but there can be many thousands of cells per drop of water, depending on whether conditions are good for algal growth or not. Algae are like plants in that they are photosynthetic. They need light and nutrients to reproduce. And there are many different species, that (just like plants) can look really different from each other.
During times of high water, when the river and lakes are hydrologically connected, and the lakes are engulfed by the river, there is little algal productivity in these lakes. This is because light is the limiting factor (or controlling factor) for phytoplankton growth in the river water. Light is the limiting factor because in river water everything algae need for growth is in abundance (i.e. there are lots of growth-promoting nutrients), except for light (their energy source). In river water the suspended sediment load is too high for light to penetrate very far (usually less than a yard), and that is pretty much true all the time for the river main channel. But in the lakes it is a different story, at least after the lake and river start to separate. As the river drops in elevation, and the connection with Modoc and other floodplain lakes decreases, the suspended sediment load (silt and sand) drops out of the water column, and (as you've seen many times) it's like turning on a switch. Pretty rapidly, the lakes turn green with the bodies of lots and lots of microscopic algae - 20 to 30 times more algal biomass per volume than in the river. So, in late spring and in summer, the lakes become green, hot spots for biological production
These algae are important sources of food for animals, from insects to fish to mammals (including the human mammal). Therefore, these lakes are critical habitat for river organisms and terrestrial organisms that can get into the lakes and start to eat what is there. I think of the lakes as nurseries or supermarkets for biological production of all kinds of organisms from the algae (at the base of the food web) to big slimy or hairy animals of various kinds. The lakes seem to stay productive (producing algae) for at least a few months after their disconnection from the river. However, as the algae accumulate they remove so many nutrients from the water, that the rate of algal production (rate at which new biomass is produced) seems to slow down. In other words, instead of being limited by light (as they were initially), their growth becomes limited by nutrient availability. But, soon enough, the river rises back into its floodplain replenishing it with deposited sediments and nutrients that will eventually support new growth. Of course, some of these nutrients have a human source (in fact, they are pollutants) - they are washed out of farm fields across the upper river basin, where they are applied as fertilizer. These backwater lakes, because they remove nutrients in the form of new algal bodies, may have some importance in removing nutrients that would otherwise flow into the Gulf. But how important the lakes are in "sequestering nutrients" (temporarily removing them from circulation) is a still unanswered question.
I should say also that as important as the connection with the river is to algal and animal growth in the lakes, the lakes are also important to the river. A lot of the algal biomass that is produced in the lakes washes back out into the river channel (paddlers sometimes see the "green plumes" of algae-rich water flowing out of backwater places), where it could be consumed by animals in the river. This may be really important, since the lower river is mostly unable to "produce its own food" - like I said before, it is too dark for photosynthesis by algae.
In freshwater ecosystems, carbon, nitrogen and phosphorus are the nutrients most needed for algal growth. Carbon (C) is not usually limiting to growth because it is plenty available dissolved in the water in the form that algae need for photosynthesis, as CO2. Nitrogen (N) and/or phosphorus (P) often limit algal growth. Both are in abundant supply in river water most of the year, except maybe in late summer, when nitrogen levels drop to a minimum. In the connected lakes, these nutrients are pulled out of the water by growing algae and other organisms, resulting in their decline in concentration over the spring-summer growing season. Nitrogen, especially, is highly elevated in the springtime due to snowmelt and spring rains carrying fertilizer into the river tributaries. As fertilizer-intensive agriculture (especially corn) spread through the Mississippi River Basin, there were increasing amounts of nitrogen that got into the river and eventually into the Gulf. This seems to be the cause of the Dead Zone (hypoxia - low oxygen) which occurs in the Gulf every year in late spring and summer.
Regarding other nutrients: There is a major group of algae, the diatoms, that are relatively abundant in river water because they are able to grow at low light levels. Like other phytoplankton, the diatoms are microscopic and planktonic (floating in the water). In addition to C, N, and P, diatoms have a pretty big nutritional requirement for silica (Si), which is used to build these amazing, intricate, and beautiful little glass cases in which the cells live. It is possible that at certain time of year Si availability can limit (control the rate of) diatom growth in the lakes, but we have not looked into that.
Scientific Abstract by Nok Pongruktham (graduate student U. Miss)
1. In the last 50-100 years, the Mississippi River has been extensively engineered for flood control and navigation purposes. These modifications have decreased the frequency of flooding and degree of contact of the river with the historical terrestrial floodplain, floodplain lakes, and other backwater sites. However, in many areas, within the levees there are still numerous secondary channels and backwater sites with which the river is connected seasonally.
2. We examined the relationship of surface hydrologic connection with the main channel of the Lower Mississippi River to limnological properties and algal biomass in three backwater sites having different patterns of hydrologic connection to the river. The backwater sites included two lakes, and a secondary channel.
3. Between November 2007 and September 2009, the depth of the river varied by up to 15 meters. At high river stage, the two lakes and the main channel were hydrologically connected. As river water flowed into or through the lakes, the lakes experienced elevated turbidity, elevated NO3-N (nitrogen) and PO4-P (phosphorous) concentrations, and had relatively low chlorophyll concentrations. As the river elevation declined, the lakes became partly or fully disconnected from the river, resulting in an increase in light as suspended sediments settled out of the water column. With this decline in turbidity, there was a rapid increase in phytoplankton biomass, and a corresponding decrease in NO3-N and PO4-P concentration.
4. Results indicate the importance of these backwater sites for production of algal organic matter, some of which may be transported into the main channel and contribute to the river food web, and possibly for nutrient immobilization, contributing to a reduction in the flux of nutrients downriver.