A Bay for All Seasons?

By H.S. Parker

For eons, the pattern has repeated. The thick, living soup of the summer Bay thins out by late fall. As winter approaches, the estuary seems to sleep. 

But Chesapeake Bay does not sleep. Even in the depths of winter, it’s poised for a new burst of life, nourished in part by nutrients from the decomposing remains of the Bay’s earlier bounty. By winter’s end, as days lengthen and Bay waters once again begin to warm, the accumulated nitrogen and phosphorous fuel a spring bloom of microscopic algae known as phytoplankton. 

Phytoplankton are vital components of marine ecosystems. While photosynthesizing, they take up copious amounts of the greenhouse gas, carbon dioxide, and give off half the world’s oxygen. They are also the primary food source for zooplankton (small, free-floating aquatic animals). Zooplankton, in turn, are preyed on by fish and crustaceans which are consumed by larger marine life forms – all the way up to the most voracious predator of all. You know who.

The warm, sheltered, well-mixed, and nutrient-rich waters of Chesapeake Bay create ideal conditions for phytoplankton growth. Over a thousand phytoplankton species inhabit the Bay, ultimately supporting most Bay life. This makes the Bay one of the most productive — and economically valuable — estuaries on earth. 

The spring phytoplankton bloom consists mainly of two forms: diatoms and dinoflagellates. The single-celled diatoms are encased in silica; they literally live in glass houses. Diatoms not only feed zooplankton, they also account for half of the oceans’ primary production. Dinoflagellates are also single-celled and favored zooplankton fare. They’re distinguished by whiplike appendages (flagella) which give them limited motility.

Supported by the phytoplankton bloom, Bay life reaches a crescendo in the summer. At that point, the diatoms and early dinoflagellates, grazed heavily by zooplankton, have been largely replaced by other algal species. By October, the vitality winds down as nutrients are depleted. In early winter, the Bay is quiet again, even as it prepares for the next cycle of rebirth and growth. 

So, like its surrounding land masses, Chesapeake Bay experiences reasonably predictable seasons. That predictability is now in question. Human activities are changing both the nature and the seasonality of the Bay’s living systems.

It comes down to the nutrients. Since the dawn of the Industrial Age, nearshore development and mechanized farming have substantially increased nutrient-laden runoff into the estuary’s waters. The excess nitrogen and phosphorous often cause abnormally large blooms of both micro- and macroalgae, far beyond the amount that can be eaten by grazers. The superabundant algae consume large amounts of oxygen, both when alive and while decomposing. This eutrophication process can result in dead zones, areas where oxygen-deficient waters no longer support most life. Eutrophication may also favor fast-growing, sometimes noxious algal species. Notably, some dinoflagellates have toxins that poison fish, shellfish, and humans. Outbreaks of such undesirable algae are called harmful algal blooms (HABs). Think red tides. 

Climate change has exacerbated eutrophication and HABs. The Bay Area has been getting warmer and wetter, with much of the increased precipitation coming in large rainfall events. Warming waters may alter the dominant algal species and affect their persistence and seasonality. Large rainstorms flush massive quantities of nutrients and sediments into the Bay. Scientists are seeing correlations between the changing Bay-area climate and a large increase of HABs.

For decades, the Chesapeake Bay Program, in partnership with many other federal, state, and regional organizations, has struggled to protect and restore the Bay. The estuary’s waters remain degraded, but we’re making progress. For example, scientists and farmers are collaborating to improve agricultural management practices that will reduce nutrient discharge into the watershed. And, throughout the region, stakeholders are establishing riparian buffer zones to restrict runoff of sediments and nutrients.

Want to help? Your best action is to advocate for robust research programs in support of the Bay’s health. We know that Chesapeake Bay is an exceedingly complicated system and that ongoing human activities confound efforts to improve the estuary’s water quality. But the more we learn about the complex factors affecting the Bay, the more likely we can effect policies to undo the damage.

Henry S. Parker is a scientist and writer who previously lived in Annapolis and now resides in Vermont.