A Typical Year
Winter (December, January, February)
Fresh water flow off the land into the tributaries of the Chesapeake Bay is usually low to moderate during winter months. This causes salinities to be slightly higher during this time of year. The water temperature is it's coldest during the winter and well mixed, making temperature, salinity and oxygen levels similar throughout the water column. Cold water holds more dissolved oxygen (DO) than warm water, so during winter months DO values are at their highest. Limited light due to shorter days and colder temperatures reduce biological activity.
The Chesapeake Bay is host to many overwintering species of organisms. Many warm water species are gone from the Bay because the surface water temperatures are too cold (below 4 degrees Celsius). Waters in the deeper tributaries, around 30 feet deep, are usually warmer than the overlying surface water. For this reason major concentrations of striped bass and white perch can be found here. From mid-February to mid-March, winter flounder spawn at the mouths of the major rivers in the Northern part of the Bay. The Bay area is home to over 75% of the waterfowl from the Atlantic Flyway, approximately one million ducks, geese, and swans, overwinter in the Bay area. The Eastern Shore is particularly important to these bird species.
Spring (March, April, May)
Things start picking up during spring months in Maryland's tributaries. As the air warms up so does the water temperature, but at a slower rate because it takes longer to heat up water than it does air. The spring is usually the rainiest time of the year in Maryland and along with the snow melt, it all runs into the Bay's tributaries lowering salinities to the minimum for the year. During early spring, the high flows keep the water well mixed and oxygenated so DO levels remain above critical levels (5 mg/l) in most areas. The high spring runoff also carries nutrients into the water, and with increased sunlight and warmer temperatures, causes algal blooms. Sediments are also carried into the Bay with high flows, and this combination of sediments and algal blooms often results in poor water clarity for periods during the spring.
This is a time of nesting and spawning for many Maryland species. With rising temperatures and minimum salinities, many of the non-resident , anadromous fish begin to enter the Bay to head for nursery or spawning areas. Much of this spawning takes place in the tidal fresh reaches of Maryland's rivers. The major spawning species are striped bass, white perch, American shad, alewife and blueback herring. Male blue crabs start to move out of their winter habitat in the deeper water to their spring habitat in the shallow water. The females begin moving up from the mouth of the Bay to the Northern part of the Bay and it's tributaries where mating will occur. Many bird species begin nesting during the spring, including the peregrine falcon and piping plover. The Bay area is also one of the mid-Atlantic regions most important heron and egret rookeries. The Chesapeake is also a nursery habitat for endangered sea turtles (American loggerhead and American Ridley) that enter the Bay this time of year.
Summer (June, July, August)
As summer air temperatures increase, the surface water temperature is the highest it will be all year. The amount of rain in the summer is usually lower than the spring and since land plants take up much of the rainfall there is often low fresh water inflow. This, along with increased evaporation, causes salinities to rise during the summer. The bottom waters are cooler and saltier than the surface water, this causes a density difference, or stratification, between the surface and bottom water. This density difference effectively keeps the bottom and the surface waters from mixing. The algae that bloomed during the spring has died and sunk to the bottom, and new algae blooms occur during summer months. As the algae decompose they use up oxygen in bottom waters. Consequently, during summer, bottom DO levels are at their lowest. We call this the summer DO sag. The oxygen levels in some of the deeper parts of tributaries, like the Potomac and Baltimore Harbor, are near zero. This is called anoxia and any kind of organism that lives on or near the bottom must leave to find oxygen or die.
During summer months, young of the year fish feed most actively and undergo their greatest growth. The Bay area becomes a gigantic nursery ground for many fish, while eagles and wading birds raise their young on the shores. The marshes and swamps are at their peak production. The young species that spawned in the spring (striped bass, white perch, etc.) begin to move downstream to saltier water. Blue crab mating actively begins in June and proceeds through October. After mating, females move to the mouth of the Bay to spawn. The summer is also an important time for oyster spawning and "setting", when oyster spat attaches to oyster rock to eventually grow into an oyster.
Fall (September, October, November)
Water temperatures start to cool down from the summer heat and approach the same levels that they were in the spring. The amount of rain usually decreases during the fall bringing even less fresh water than during the summer, thereby leading to increasing salinities. As the water temperature drops the density difference between the bottom and surface waters lessens and the two layers begin to mix; stronger fall winds help in mixing the waters, bringing oxygenated surface water to the bottom, resulting in increasing bottom DO levels. The decreased fresh water inflow brings in less sediments and nutrients leading to generally improved water clarity.
The fall is a transition period for the organisms of the Bay. The population of warm-water species gives way to those that use the Bay as an overwintering area. Young striped bass, white perch and spot begin to move farther downstream into deeper water where they will overwinter. Male blue crabs also head for deeper water and by October mating has stopped. Oyster spawning and spat setting continue into the early fall. Many bird species fly in from the north to overwinter here in the Bay area.