Fibrujry 1964 



April 



Incidence of Gymnodlnium breve 



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July 



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Figure 3. — Incidence of G. breve (percentage of total 

 samples containing the organism) by month and distance 

 from shore along the west coast of Florida, February 

 1964 to February 1965. Data are combined for Tampa 

 Bay and Charlotte Harbor, and for the four series of 

 stations shown in figure 1. Solid black circle is equal 

 to 100 percent. 



ENVIRONMENTAL FACTORS 



The distribution and abundance of marine 

 phytoplankton are influenced by temperature 

 and salinity. The geographic distribution of 

 phytoplankton is governed by temperature. 

 The latitudinal occurrence of phytoplankton in 

 neritic and estuarine areas is controlled pri- 

 marily by salinity fluctuations which, in turn, 

 are influenced by precipitation. 



Temperature 



Maximum water temperature (31.1° C.) was 

 observed in August at Charlotte Harbor, and 



the minimum (13.8° C.) in February, 27.8 km. 

 (15 miles) off Johns Pass, near the bottom. 

 Low temperatures extended from December 

 through March in most of the area (table 1). 

 Warming began in March, and temperatures 

 were highest in July and August. Temperature 

 decreased in most areas from September 

 through December. Off Johns Pass this decline 

 continued through February 1965 and off Anna 

 Maria through January. 



Thermal gradients were negative in March 

 and September, 27.8 km. (15 miles) and 37.1 

 km. (20 miles) offshore. The maximum gradient 

 (3.7° C.) occurred in September, 37.1 km, 

 (20 miles) offshore. For the remaining months, 

 vertical variations of temperature never ex- 

 ceeded 1.6° C, but they were greater than 

 1° C. 15 times. 



Precipitation 



Heavy rainfall is often given as a likely 

 factor in the production of red tide (Numann, 

 1957). Although it has not always been asso- 

 ciated with outbreaks of Florida red tide, it 

 must be considered as one of the factors 

 favoring red-tide outbreaks. 



Precipitation during 1964 at St. Petersburg, 

 Bradenton, Punta Gorda, and Fort Myers was 

 well below the long-term means, although it 

 was 16.15 cm. (6.36 inches) above the mean at 

 Tampa (table 2). The lack of red-tide out- 

 breaks during the present study was coinci- 

 dental with the relatively low rainfall. 



Salinity 



The maximum salinity (37.10 p.p.t.) was 20 

 miles off Venice Inlet, near the bottom, in 

 July; the minimum (17.83 p.p.t.) was in Hills- 

 borough Bay, at the surface in April. Seasonal 

 changes of salinity are similar at all offshore 

 stations (fig. 4). Monthly changes in salinity 

 were nnore pronounced at stations 5 and 10 

 miles offshore than at 15 and 20 miles. Salinity 

 increased steadily at most stations from 

 February through July and began to decline 

 in August during the rainy season. It in- 

 creased in November but remained below 

 the maximum summer value from November 

 through February. 



In terms of distance from shore, salinity 

 increased progressively from 5 to 20 miles 

 offshore at all transects only in April and 

 December (fig. 4), Existence of water masses 

 with reduced salinity at stations 20 miles 

 offshore was particularly obvious in Septem- 

 ber. 



Salinity increased with increasing depth at 

 offshore stations throughout most of the in- 

 vestigation.. The gradient exceeded 1 p.p.t. 

 during February, June, August, September, 

 October, and November. 



