A woman running is breathing in oxygen and releasing carbon dioxide pretty quickly. This is true especially compared to a bystander like me at last week's NY City Marathon. No matter how loudly I cheered, my metabolic activity was much lower than a runner's. This explains why I chose to dress in a down jacket whereas some of the runners were wearing little more than a bathing suit.
On a larger scale, ecosystems breathe at different rates as well. The primary producers in the ecosystem take in carbon dioxide and release oxygen while making sugars via photosynthesis. The consumers do the opposite, taking in oxygen and releasing carbon dioxide while harvesting the energy stored in sugars, fats and proteins via respiration. The metabolic activity of both producers and consumers can vary, resulting in different breathing rates among ecosystems.
We can see this variation if we examine carbon dioxide and dissolved oxygen concentrations in three different ecosystems in the Hudson River. Very active systems will have a broader spread between high and low dissolved oxygen and carbon dioxide concentrations, compared to less active systems. This is because producers are only active during the day whereas consumers are active day and night. Also, the tides are continually flushing the systems, resetting the conditions every twelve hours.
The HRECOS system does not monitor carbon dioxide concentrations. However, since this molecule forms carbonic acid when mixed with water, we can use pH to observe changes in concentrations of carbon dioxide. The relationship is not perfect but generally, as CO2 concentrations increase, pH decreases and as CO2 concentrations decrease, pH increases.
The HRECOS station at Norrie Point measures water from the main stem of the river. There are plants, animals, and microbes at this station, but they are not as dense and the gases from their breathing are diluted in a larger volume of water. Dissolved oxygen concentrations and pH fluctuate, but not broadly. In other words, the metabolic activity at Norrie Point is low. Norrie Point is like me standing on the sidelines at the marathon.
Compared to Norrie Point, the submerged aquatic vegetation at Esopus Meadows is a hotbed of activity. The most abundant producer in this ecosystem is Vallisneria americana, also known as water celery and is capable of very high rates of oxygen production. Consumers in this habitat include micro-organisms, snails, mussels, flatworms, insects and the fish such as sunfish, eels, silversides, herrings, carp, killifish, sticklebacks, bass and perch (Strayer 2006; Findlay et al., 2006). In addition, the plants also act as consumers when they make use of the sugars they produce.
In order to measure dissolved oxygen concentrations and pH at this site, Stuart Findlay of the Cary Institute installed a temporary monitoring instrument. This effort was part of a larger study to examine the function of wetlands and how they may be affected by man-made changes (Findlay et al, 2002).
Dissolved oxygen concentrations and pH fluctuate more broadly in the submerged aquatic vegetation bed than they do at the HRECOS station at Norrie Point illustrating the difference in metabolic activity between these two sites. The plants and consumers in the submerged aquatic vegetation are like the river of runners who passed me by in the NY City Marathon.
A third system for comparison is provided by the HRECOS station located at Tivoli Bay North, an intertidal marsh located at the south of the Village of Tivoli where consumers greatly outnumber producers. Consumers in this marsh include micro-organisms, insects, worms and fish such as bass, perch, and common carp. The difference is that most of the producers in Tivoli Bay North take and release gas from the air and not the water. Spatterdock, pickerel-weed, narrow-leaved cattail, purple loosestrife, and common reed are rooted under water but have raised their leaves above water. Only the few submerged aquatic plants such as water celery are contributing and removing gases to and from the system (Tivoli Bays - Hudson River Reserve, 2004).
Measurements of dissolved oxygen and pH vary broadly at Tivoli Bay North but the range is less than what is observed in the submerged aquatic vegetation at Esopus Meadows. The consumers are using more oxygen than is being produced and releasing more carbon dioxide than is being removed. Luckily for the consumers in the North Tivoli Bay Marsh, the system is flushed every twelve hours by the tidal cycle. Water from more productive sections of the river is pushed in and replenishes the oxygen supply. Without this restart, animals in the marsh would suffer from anoxia, a lack of oxygen. Like a runner who suffers from cramping muscles from low oxygen supply, the system would start to break down.
The varying contributions of the producers and consumers in these ecosystems may change over the next fifty years. Global warming will result in higher water levels in the Hudson River. Since many of the submerged plants can only survive in waters that are less than three meters deep, the producer communities in these systems may change (see this simulation of what may happen at Tivoli Bay North produced by the Changing Hudson Project). High frequency monitoring such as HRECOS will be a necessary tool for monitoring these changes and their impacts on other ecosystems in the Hudson River Estuary.
Findlay, S., C. Wigand, and W.C. Nieder. 2006. Submersed Macrophyte Distribution and Function in the Tidal Freshwater Hudson River. pp. 230-241 in J.S. Levinton and J.R. Waldman (eds.). The Hudson River Estuary. Cambridge University Press, NY.
Kiviat, E., S. Findlay, and W.C. Nieder. 2006. Tidal Wetlands of the Hudson River Estuary. pp. 279-295 in J.S. Levinton and J.R. Waldman (eds.). The Hudson River Estuary. Cambridge University Press, NY.