Bay Scientist Tom Parham takes us through the Chesapeake seasons.
Our yourself some hot coffee and watch how a bit of cold, dense cream plummets to the bottom of the cup. The coffee and cream remain separated until you stir it with a spoon.
This is similar to how Chesapeake Bay waters behave through the seasons. Fresh water and salt water remain separate until conditions are right. Once those conditions are met, major mixing occurs.
The technical definition of an estuary is “a semi-enclosed coastal body of water with a free connection to the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.” This sounds straight forward. But add in how changing temperatures, rainfall and wind impact the Bay’s mixing processes throughout the seasons in 180 miles of water (18 trillion gallons) from Havre de Grace to the Bay Bridge tunnel and we find that the Chesapeake Bay is a dynamic system as varied as the plants and animals that call it home.
The Bay is a mixing bowl where 150 major rivers and streams run off its 64,000 square mile watershed to meet the ocean water entering the Bay’s mouth near Norfolk. The Bay’s salinity is lowest at the top of the Bay at Havre de Grace, where the mighty Susquehanna River drains rain runoff and snowmelt from as far away as Cooperstown, New York. Bay water salinity increases towards the mouth of the Bay, where tidal ocean waters flow in and out. More rain results in more river flow, pushing saltwater down the Bay. Less rain allows the salty ocean waters to creep north. Salinity also increases with depth. Fresh water is more buoyant than salt water—therefore, the fresh water flowing down the rivers tends to float on top of the denser salt water moving its way up the Bay.
At the northern, upper Bay edge of the salt water front, the salt water and fresh develop a mixing zone that re-suspends nutrients and other important materials for Bay organisms. Rich in food, these areas are critical habitats for many species of juvenile fish and other Bay denizens. Larger predator fish such as striped bass stalk these waters to feast on the abundant variety of prey. The location of this zone varies depending on flow and wind conditions. Fishing tip: It is typically found just north of the Bay Bridge up to a line near the Bush River.
In the Bay, fresh and salt waters mix because of the wind, river flow, tidal currents and the shape of the Bay. Temperature is also a major factor. Warm water is more buoyant than cool water, so the greater the temperature and salinity differences between these water masses, the less mixing takes place. Major mixing occurs when the surface waters cool and the density of surface and bottom waters become more similar. In the Bay, this cooling of waters aided by strong, sustained winds is like is the spoon that stirs the coffee.
So, pour yourself a cup of coffee and let’s take a trip around the estuary to see how seasonal changes affect some of the fish, birds, shellfish and the underwater plants of the Chesapeake Bay.
Bay waters begin to warm in mid-March through mid-May as daylight hours increase nearly 35 percent from winter conditions. The shallow rivers warm first, but the Bay waters are still cool, with oxygen plentiful throughout. Runoff into the Bay increases from rainfall and snowmelt causing surface salinities to decrease and pushing fresh water further down the Bay. These flows also carry suspended sediment and algal bloom-fueling nutrients from the land, which cloud the water and result in reduced water clarity. Increased freshwater flow moving down the Bay causes the denser, higher-salinity bottom waters to be pulled up from the Bay’s mouth, establishing the Bay’s two-layer flow.
Warming waters also increase the Bay’s animal and aquatic plant activity. Usually in May, water temperatures warm enough to wake the blue crabs from their winter slumber along the muddy channel edges. Male and female crabs emerge and move to shallower waters in the upper portions of the Bay and rivers to feed and prepare to mate. For the oyster, warm water and abundant algal food increase its feeding rate, which will help grow new shell. Striped bass join the waves of American and hickory shad, blueback herring, alewife, and white perch moving up the Bay to spawn in the fresh or low salinity waters. Spring also marks the return of the osprey to the Chesapeake Bay to feed on abundant spawning fish. Most underwater grasses, also known as submerged aquatic vegetation, first appear in April in the shallow, warmer areas of the Bay. These key aquatic plants provide food and shelter for waterfowl, fish, shellfish and invertebrates. In addition, they produce oxygen, filter the water, and absorb excess nutrients from waters.
Bay waters reach maximum temperature in mid-June through mid-August as daylight hours increase nearly 40 percent from winter conditions. The Bay’s waters and its rivers become warmer, blanketing the cooler waters in the depths of the main Bay and near the Bay mouth. Summer’s freshwater flows into the Bay are lower due to increased evaporation and retention of rainfall by growing plants and trees. As a result, the Bay becomes saltier.
The warmer water temperature also increases the Bay’s biological activity. Underwater grasses now carpet shallow areas and provide habitat for juvenile fish and crabs. Warm, sunny days fuel algal growth throughout the Bay and reduce water clarity. When excessive amounts of algae die, settle to the bottom, and decompose, the process removes large amounts of oxygen. This worsens conditions in existing low oxygen areas, which occur in the deeper, denser, oxygen-poor, saltier waters, and creates the “dead zones,” which are inhospitable and even deadly for fish, crabs, and bottom dwellers such as shellfish.
Each summer, these dead zones occur in large portions of the deeper waters from the Bay Bridge southward nearly to Tangier Island. For fish seeking cooler waters, these deeper waters provide a respite from warm surface temperatures—but, because of the dead zones, they are often oxygen-poor. Striped bass and other fish are squeezed into small areas that are cool enough and provide adequate oxygen. While anglers can have incredible fishing in these small pockets of trapped fish, it is important to understand that these water conditions are very stressful for striped bass. To help reduce dead zones, a primary Bay restoration goal is to reduce the excess nutrients that spur algal growth. The less algae, the less dead zones and the more oxygenated areas for fish, crabs and shellfish to thrive.
During the summer, gentle south winds dominate the Bay. However, periods of stronger, sustained winds from the north or south can temporarily increase the mixing of buoyant surface waters and the dense bottom waters, recharging some of oxygen in deeper waters.
As summer progresses, striped bass, white perch and other fish that spawned in the spring start to move down river to saltier, cooler waters. Osprey feed on these abundant fish until they fly south in late August. Blue crabs mate through the summer, then the sexes separate, with the male crabs moving up the Bay towards fresher waters while the females move towards the Bay’s mouth to spawn. Now, oysters are spawning and the free swimming larvae seek places to settle and become spat.
From mid-September through mid-November, Bay waters begin cooling as the days shorten. Freshwater flows and the sediments and nutrients they carry decrease, resulting in higher Bay salinities and improved water clarity.
The Bay surface waters and the rivers cool first, edging the water density closer towards that of the bottom waters. Many fish move from the cooling rivers to the warmer main Bay. By November, the density differences are small enough that when combined with strong, sustained northwest winds, the water column mixes from top to bottom—recharging the waters with oxygen from the surface to the bottom to the benefit of fish, crabs and shellfish. Mixing also brings nutrients to the surface waters for use by algae and other organisms.
As Bay waters continue to cool in the early fall, blue crabs move to deeper channels to bury in the mud for the winter. Oyster spawning and oyster larvae settling remains active. By late fall, conditions are prime for oyster growth—spawning is over, waters are cooler, and food is abundant. After the Bay mixes and the surface waters of the main Bay continue to chill, resident striped bass move towards the deeper, warmer, bottom waters where they overwinter. On the Bay’s bottom, most underwater grasses prepare for the end of the growing season by producing seeds or tubers. Eelgrass is the exception as it thrives throughout the cooler conditions in the higher-salinity waters from the Tangier Sound region southward to the mouth of the Bay.
Due to the short daylight hours from mid-December through mid-February, Bay waters become cold. Rainfall is low and moisture may become locked in place as snow and ice, reducing the amount of suspended sediments reaching the Bay and increasing water clarity. Freshwater flows decline, allowing saltier water to move up the Bay. The prevailing winter wind is often from the west, but sometimes strong, prolonged, northwest or north winds will churn the cold winter waters. At river mouths and deeper parts of the Bay, remaining fish may overwinter in the slightly warmer bottom waters, protected from strong currents. Blue crabs are buried in mud along the edges of deeper channels, often adjusting their depth as the water warms and cools minutely. By December, the oysters are fat with stored-up energy for the winter, and their growth attenuates as the water cools and algae becomes less abundant. Eelgrass, the only submerged aquatic grass growing during the winter season, provides fodder for overwintering waterfowl as they fatten up for their northern migration.
So, the next time you want to consider how the Chesapeake Bay changes over the seasons, start your conversation with a cup of coffee and let it guide your imagination toward the Bay’s constant transformatons.
Tom Parham oversees the Maryland Department of Natural Resources Tidal Water Monitoring Program, and he’s an avid angler.