(Posted by Bill Dennison.)
Chesapeake Bay is particularly vulnerable to human impacts and incredibly productive for three basic reasons. 1) The first of these reasons is that Chesapeake Bay is an estuary, a place where fresh and salt water mix. 2) The second of these reasons is due to the geographic setting; geomorphology and hydrography of the Bay and the human footprint on its surrounding lands. 3) And the third reason has to due with the various nutrient and particle retention processes that occur with Chesapeake Bay. Each of these reasons will be explained further, but the net result is that Chesapeake Bay is particularly sensitive to overenrichment of nutrients, as well as sediment and toxicant runoff. In the case of Chesapeake Bay, vulnerability is the flip side of productivity. For all the same reasons that Chesapeake Bay is vulnerable to human impacts, the Bay has been and remains incredibly productive in terms of primary productivity (sunlight converted into plant life) and secondary productivity (plant life converted into animal life).
1) Chesapeake Bay is a large estuary. There are several features of estuaries that make them productive and vulnerable. One process occurs at low salinities where there is a physical-chemical process that leads to the precipitation of various dissolved compounds due to the salt content of the water, often leading to an estuarine turbidity maximum. The estuarine turbidity maximum (known as the ETM) entrains nutrients and particles and various organisms cue in on this location. Another feature of estuaries that makes them both productive and vulnerable is the circulation pattern caused by two-layer flow. Freshwater derived from river flow into the estuary floats on top of salt water derived from the oceanic flow into the estuary. As surface water moves down estuary, bottom water moves up estuary. Any particles formed in surface waters that sink into the bottom waters will tend to be pushed back up estuary. This conveyor belt of surface flow downstream and bottom flow upstream leads to better retention of materials in the estuary. Another feature of estuaries that enhances productivity is the energy subsidy that occurs with tidal pumping. The twice-daily tides in Chesapeake Bay causes water to flow in both directions repeatedly. Even fresh water regions can be influenced by tides—these regions are known as tidal freshwater and the tidal freshwater marshes of Chesapeake Bay have been shown to be incredibly productive and capable of massive nutrient absorption.
2) Chesapeake Bay is very shallow, has an extensive shoreline and the adjacent watershed has a substantial human footprint. The average depth of Chesapeake Bay is less than 25 feet, with vast reaches that are only several feet deep. Thus, nutrient cycling between sediments and the overlying water is very efficient. Sunlight can reach the bottom in much of Chesapeake Bay, particularly when historical water clarity was higher than current conditions. Microscopic and macroscopic plants can thrive where nutrients from sediments and light from the sun are simultaneously available. The Bay was formed from drowned river valleys associated with sea level rise over the past several thousand years. The convoluted nature of the shoreline reflects this geologic history, and there are literally thousands of miles of shoreline in Chesapeake Bay. This land/sea interface is an ‘ecotone’, the productive intersection of different habitats. The human footprint in the Chesapeake Bay watershed has been extensive from the times of native American settlement through European settlement. Rich agricultural soils, abundant freshwater resources, and access to the Bay for navigation and trade, has made this region consistently attractive for human settlement.
3) Chesapeake Bay is nutrient, sediment and toxicant retentive. There are several oceanographic processes that serve to reduce the flushing of substances out of Chesapeake Bay into the Atlantic Ocean, described by Mike Roman and colleagues (Limnology and Oceanography, 2005). The estuarine turbidity maximum (ETM), described above, occurs in low salinity regions in the upper Bay and upper portions of the tributaries. The river plume fronts that occur at higher salinities where the tributaries discharge into the mainstem of the Bay are also regions where nutrients, particles and organisms collect. The Bay is so large near the mouth that a small version of the large circular circulation gyres that occur in the ocean actually occurs within the Bay. This slow moving convergence eddy effectively retains water and particles. The hydrography of the Bay in the northern Virginia portion involves the movement of water from deeper areas of the Bay into shallow water, creating a hydraulic control in which the water flow is reversed. Finally, the tidal fronts that occur near the mouth of the Bay as the incoming and outgoing tides meet and form vertical walls of current shear are locations where nutrients, particles and organism collect. Depending on the river flow of a given year, the ETM may be extensive one year, but hydraulic control and the convergence eddy may be more important in another year. This compensation of various retention mechanisms means that the Bay is essentially always retentive.
These features that make Chesapeake Bay both productive and vulnerable increase the onus on our management of the Bay to be particularly vigilant and effective. More than most coastal regions, Chesapeake Bay management needs to insure that the loads of nutrients, sediments and toxicants are reduced so that this productive but vulnerable Bay is not further degraded.