Bay Phytoplankton Conditions - 1998-2002
We have observed tremendous inter-annual variability of phytoplankton distributions in the Bay during the life of CBRSP. Much of the variability can be traced to the variable rainfall in the Bay’s watershed and the impact on freshwater input and nutrient loading from the Susquehanna River and other large rivers and tributaries feeding the estuary. In Chesapeake Bay, the spring bloom dominates the processing of nutrients and affects the subsequent summer conditions by providing the substrate for nutrient regeneration to drive further algal production. In years of high rainfall and high river flow (1993, 1994, 1996, 1998, 2000), nutrient loads and suspended sediment carried in the freshwater flow influence the nutrient and light conditions in the receiving waters along the entire main stem of the Bay. Algal abundances in high-flow years were very high, particularly during the spring bloom period, April to mid-May, and highest chlorophyll concentrations were consistently found seaward of the peaks observed in moderate or low flow years. In years of low rainfall and low river flow (1995, 1999, 2002), nutrient delivery is much reduced and phytoplankton biomass in the main stem of the Bay declines. It appears that the relative position of the chlorophyll maximum is related to the magnitude of flow because nutrient inputs during spring when freshwater flow is maximal are largely derived from the Susquehanna River at the head of the Bay.
The aircraft remote sensing program using ODAS and SAS II / III has given us a detailed view of phytoplankton dynamics in the Chesapeake Bay. The following paragraphs summarize our findings for the past five years.
1998 was another year of high spring flow, reminiscent of 1996 but not quite so extreme. A robust phytoplankton bloom developed in the mid- to lower Bay, exemplified by the chlorophyll image of 21 April 1998. The high freshwater flow from the Susquehanna River in spring was followed by a prolonged summer drought, with lower nutrient inputs to the Bay. Relatively high summer chlorophyll concentrations were observed despite the low flow, fueled by regenerated material produced in the spring, as shown in the chlorophyll images for 24 July and 3 August 1998.
1999 was characterized by a strong, regional drought and freshwater flow was very low, affecting nutrient loading and resulting in much-reduced phytoplankton abundance that was observed in the remotely sensed observations as low concentrations of chlorophyll. These conditions persisted throughout early spring, as shown in imagery for 6 April to 28 April 1999. Despite the low freshwater flow, conditions reversed in May and high chlorophyll concentrations were detected in much of the Bay, shown in images for 11 May and 21 May 1999. The image of 9 June 1999. shows that this condition persisted into early summer. By mid-June chlorophyll declined and relatively low concentrations were observed for the balance of summer.
2000 was characterized by high freshwater flow and phytoplankton blooms were fairly well developed at a number of locations in the Bay. Images for April and May 2000 show chlorophyll concentrations from 10 to 30 mg m-3covering large areas of the Bay. 14 May 2000 shows high chlorophyll > 30 mg m-3 in the mid to upper Bay that coincided with dinoflagellates of the common Bay species, Prorocentrum minimum, that forms annual blooms in this region. Seaward peaks of chlorophyll were associated with abundant diatoms of a variety of species, as usual. By early June, chlorophyll concentrations had declined appreciably during a floral transition to summer forms. Chlorophyll concentrations increased in late June to early July 2000, particularly along the western shore of the mid- to upper Bay between Annapolis and the mouth of the Patuxent River.
2001 is a fairly average year in terms of freshwater flow and phytoplankton abundance. Images from late April to mid-May 2001 show elevated chlorophyll concentrations in the mid-Bay, and lower concentrations toward the mouth of the Bay. Episodic peaks of chlorophyll in the upper Bay were observed 24 May 2001, typical of the seasonal dinoflagellate blooms commonly observed in this region over the past 25 years. Summer chlorophyll declined appreciably from the late spring peaks, with some localized peaks in the upper Bay. We had an interruption of data collection following the 11 September 2001 terrorist attacks on the World Trade Center and Pentagon, but have resumed flights with some limitations on our coverage imposed by security concerns. It is our plan to continue flights into December at a reduced frequency, and to resume Bay-wide coverage in 2002.
2002 was a strong drought year with low delivery of freshwater and nutrients to the main stem of the Bay from the Susquehanna R. and other main riverine sources. These conditions resulted in low chlorophyll during spring that revealed a much reduced “spring bloom”. Images for 6 March, 28 March, 16 April, 24 April and 30 April 2002 show concentrations < 6 mg m-3 throughout the mesohaline to polyhaline regions of the Bay, i.e., seaward of the Patuxent R. mouth. Some localized increases of chlorophyll to concentrations > 15 mg m-3 occurred in the upper Bay, as shown in images for 16 April, 24 April, and 30 April 2002 near the Patapsco R. mouth. Summer chlorophyll was also low in much of the Bay, with the exception of the oligohaline and upper mesohaline regions where concentrations > 20 mg m-3 occurred in July and August, between the mouths of the Patuxent and Patapsco R. Chlorophyll declined in fall throughout the Bay.
