Oxygen isn’t important only for air-breathing land animals; fish and other in-water residents need sufficient oxygen in the water—“dissolved oxygen”—to survive.
While the level of oxygen needed varies from species to species, dissolved oxygen levels below 5 milligrams/liter begin to stress the Bay’s inhabitants. Low levels are troublesome for striped bass, blue crabs, and especially for oysters—who can’t relocate to look for water with more oxygen.
In general, August experiences oxygen levels lower than other times of the year and this August is starting off with dangerously low levels. August 2011 is setting up to have extremely high levels of hypoxia—where oxygen levels are below 2 milligrams/liter—and anoxia—less than 0.2 milligrams/liter.
Different things can affect the dissolved oxygen levels in the Bay: How much fresh water flowed into the Bay in springtime—and how many nutrients did it carry with it? How well is the Bay circulating—bringing water to the surface where it can “breathe” more oxygen? What is the water temperature in the Bay?
All these variables have an effect on a major contributor to low dissolved oxygen: algae blooms. High levels of springtime freshwater flow into the Bay bring in nutrients and also lead to a strongly stratified vertical structure. The hot summer sun warms the surface waters, further increasing stratification and fueling photosynthesis. In the sunlit near-surface waters, the tiny algae plants – phytoplankton – use the plentiful nutrients to grow to unsustainable populations. While their photosynthesis produce s oxygen during the daylight hours, it is trapped near the surface by the stratification, and cannot replenish the bottom waters. The algae die quickly, falling to the deeper waters, where their decomposition process further uses up any available oxygen. Wind events energetic enough to mix the surface oxygen to the bottom rarely happen in the summer, and the cycle continues to increase the volume of oxygen-depleted waters. This year, record heat waves, high spring river flows, and lack of significant wind events are working together against Chesapeake Bay marine life.
So how can we track dissolved oxygen levels in the Bay, which in turn help us monitor habitat availability for the Bay’s fish, shellfish, and crabs? Scientists at NOAA are using the NOAA Chesapeake Bay Interpretive Buoy System (CBIBS) to continually monitor water quality, including oxygen levels, at nine buoys located through the Bay. The CBIBS buoys also report water temperature, salinity, and chlorophyll-a, which help scientists identify hypoxia.
For example, the first graph illustrates the level of dissolved oxygen available to fish and crabs just under the surface of the water at the Annapolis buoy over the early summer this year. Graphs show percent saturation – since the average saturation value during summer in Annapolis is around 7 milligrams/liter, our hypoxia level is around 30%. These are daily average values along with the minimum value during each day, removing daily fluctuations due to photosynthetic activity.
The second graph shows similar data from an instrument on the bottom, at around 35 feet, at the CBIBS Gooses Reef buoy location (the Gooses Reef buoy is the only location so far to have an additional sensor positioned on the Bay bottom). Since late May, there has been basically no oxygen available here for living creatures. This represents both an early onset and relentless continuation of anoxic bottom conditions.
While the overall trend in the graphs is downward, there are notable fluctuations. Every day, there can be changes in the oxygen level based on the life and death cycle of algae, fueled by sunlight.
In addition to CBIBS data, the NOAA Chesapeake Bay Office is collaborating with agencies and institutions around the Bay including the Maryland Department of Natural Resources (DNR) to compile as much data as possible on hypoxia in the Bay. The combination of data from fixed platforms like buoys with snapshots of hypoxia from research cruises on the Bay can be valuable to scientists as they attempt to better understand hypoxia and anoxia in the Bay. Looking to the future, the NOAA Chesapeake Bay Office, along with the NOAA National Weather Service and NOAA National Ocean Service , are discussing ways of producing ecological forecasts using computer models of the Bay ecosystem, to include forecasts of oxygen levels. Adding real data into their computer models will result in more accurate forecasts.
Be sure to check back for future articles on Bay conditions. What dissolved oxygen levels are fish and crabs experiencing at locations near you?