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River Plume Productivity Relationship with the Spring Bloom & Fall Bloom of Dinoflagellates
A close examination of the Coastal Zone Color Scanner (CZCS) data shows the episodic formation of a high pigment plume in the northern part of the West Florida Shelf during spring. The degradation of this spring bloom to organic and inorganic materials may lead to subsequent fall blooms of dinoflagellates within the inner shelf waters of central Florida. Gilbes et al. (1996) have observed this phytoplankton plume, which has remained undetected by other investigators as a result of infrequent ship surveys. It extends southward from Cape San Blas toward the Florida Keys along the shelf break (Figure1). The regular occurrence of this plume may support the idea that energy transfer to higher trophic levels on the northern part of the West Florida Shelf is seasonal.
Occurrence and Lifetime of the West Florida Plume:
In 1979, the West Florida plume occurred during March, and it lasted at least two weeks (Figure 2). The plume was first detected on March 9, when pigment concentrations were between 0.5 and 1.0 mg/m³. On March 15, the plume extended southward from Cape San Blas along the shelf with pigment values between 2.0 and 5.5 mg/m³. Pigment concentrations were between 1.0 and 2.0 mg/m³ on March 20 and < 0.5 mg/m³ on April 1.
In 1980, the plume on the West Florida Shelf had maximum concentrations in May and a duration of approximately one month (Figure 3). The plume was first detected off Cape San Blas on April 24, with pigment concentrations >0.5 mg/m³, while further south the pigment concentration remained low (0.1-0.2 mg/m³). On May 16, a well-defined plume extended southward with values up to 5.0 mg/m³. On May 19, pigment concentrations decreased to < 2.0 mg/m³. The plume disappeared on June 2 (< 0.25 mg/m³).
Similar plumes on the West Florida Shelf were detected in CZCS imagery between 1981 and 1986. However, the number of usable images during these years was lower than in previous years, making the analysis of the time sequence of the plumes more difficult. A reduction in CZCS operational time plus cloud cover were the major reasons for decreased coverage. A plume with pigment concentrations >0.5 mg/m³ was identified in February 1981, February 1982, May 1983, April 1984, May-June 1985, and March 1986. The duration of the plume for those years varied from one week to one month.
In 1986, the maximum concentrations of the West Florida plume was observed in March and it lasted about a month (Figure 4). In mid-January coastal waters off Cape San Blas began showing elevated pigment concentrations. On March 3, a plume occurred along the West Florida Shelf with pigment concentrations >0.5 mg/m³. On March 7, the plume extended southward and pigment values exceeded 1.0 mg/m³. Pigment concentrations were lower by April 1.
Processes Responsible for the Formation of the West Florida Plume:
While the periodic occurrence of the chlorophyll plumes is confirmed by in situ and satellite data, the processes responsible for the formation of these plumes remain unclear. Gilbes et al. (1996) found that such plumes may be related to one or a combination of the following factors:
The Gulf coast of Florida receives the discharge of 21 rivers, primarily concentrated in the northern region. Although it has been demonstrated that the runoff of these rivers affects the chemistry and biology of estuarine zones, their impact on shelf waters is unclear. The northwest Florida rivers (Perdido, Escambia, Blackwater, Yellow, Choctawhatchee, Econfina Creek, Apalachicola, Ochlockonee, Aucilla, Encofina, Fenholloway, Steinhatchee and Suwannee Rivers) showed maximum and minimum discharge during spring and fall, respectively (Figure 5). Peak discharge during spring coincided with the appearance of the West Florida plumes in the CZCS images. The southwest Florida rivers (Withlacoochee, Hillsborough, Alafia, Little Manatee, Manatee, Myakka and Peace Rivers) showed more irregularity, but in general the maximum discharge was during summer-fall (Figure 5). The high seasonal discharge of the northwestern rivers can represent a source of inorganic nutrients for the formation of the West Florida plume. Gilbes et al. (1996) examined the inorganic nitrogen loading for the Apalachicola, Suwannee, Escambia, and Choctawhatchee Rivers. The inorganic nitrogen loading during winter-spring (January to June) was about two times higher than during summer-fall (July to December). High pigment concentrations (>0.5 mg/m³) at northwest Florida shelf showed a correspondence with high nitrogen loading during March 1979, November 1980, and March 1981.
A recent study suggests that southward circulation on the West Florida Shelf is produced by differential heating and cooling within different regions (Robert Weisberg, personal communication). The Loop Current has a significant year-round effect on maintaining a higher temperature offshore relative to waters on the West Florida Shelf. Calculations of steric height differences show that the sea level at a shelf location can be between 27 and 33 cm lower than an offshore location at the same latitude. These differences generate a pressure gradient toward the shore that will be balanced by Coriolis. Assuming a barotropic condition, the water mass will therefore move southward, with the strength of the flow proportional to the differences in steric height. Southward movement of shelf waters may transport phytoplankton, nutrients, dissolved organic matter, and suspended sediments from northern to central and southern regions. As this effect would have a maximum in the February – April timeframe, coinciding with the spring maximum discharge of rivers into the Northern Gulf, the southward plumes detected along the West Florida Shelf may be caused by this type of circulation.
Loop Current Intrusions:
Nutrient supply associated with upwelling caused by intrusion of the Loop Current to the Southwest Florida Shelf is restricted to the outer shelf. In contrast, on the Northwest Florida Shelf, large intrusions of the Loop Current affect shelf waters to within a few kilometers of the beach. Some researchers have suggested that "the canyon may have been a conduit for the northward flow of deep gulf waters." Previous observations suggest that an episodic upwelling through the De Soto Canyon may provide nutrient injections to the inner West Florida Shelf from deep offshore waters. During January and February of 1992, the Loop Current intruded as far as 29° North into the Gulf of Mexico. This intrusion could thus have reached the DeSoto Canyon, supplying nutrients from deep waters. The phytoplankton communities of Northwest Florida may have responded to this input during March 1992 (Figure 1). However, there is insufficient information to quantitatively evaluate the role of the DeSoto Canyon and the Loop Current on episodic nutrient supply; more research is required.
Mississippi and Mobile Rivers' Discharge:
The discharge of the Mississippi and Mobile Rivers represents a possible source of "Colored Organic Matter (COM)" on the West Florida Shelf. The Mississippi River ranged from 4,932 to 45,960 m³/s and the Mobile River ranged from 282 to 9,018 m³/s during 1979-1985 (Figure 5). Both rivers showed high and low discharge during spring and summer, respectively. The high discharge in spring coincides with high pigment concentrations and the occurrence of the West Florida plumes detected by the CZCS at that time. However, the seasonal correspondence between discharge and pigments was not as good as with the northwest Florida rivers. Several studies show that the Mississippi River outflow can move toward the southeast and become entrained in the eastern edge of the Loop Current. The episodic northward intrusions of the Loop Current during winter and spring can transport low-salinity and chlorophyll a/COM rich waters from the Mississippi and Mobile Rivers to the West Florida Shelf. This occasional eastward transport to the West Florida Shelf may be added to the input of local rivers and Loop Current induced upwelling, all producing good conditions for phytoplankton growth and biomass accumulation.
The complexity of the West Florida Shelf has been demonstrated with the CZCS and field data. An initial diatom spring bloom may originate in waters off northwest Florida, due mainly to the input of nutrients from local river discharge. Intrusions of the Loop Current may induce both upwelling in the DeSoto Canyon and eastward transport of effluents from the Mississippi and Mobile Rivers. Shelf circulation patterns induced by steric height differences can move the phytoplankton and other suspended organic material southward. Still, these processes are poorly understood. Future studies in the West Florida Shelf must provide more spatial and temporal field data in order to elucidate the relationship between phytoplankton processes, dissolved organic carbon, inherent and apparent optical properties, and water circulation. The next generation of ocean color sensors will provide the synoptic view.