sponsible for the greater portion of total phos- 

 phorus. The quantities of phosphate-phosphorus 

 always exceeded the minimum necessary for the 

 growth of phytoplankters in vitro (Ketchum, 

 1939). 



IRON 



The study of the distribution of iron in Florida's 

 west coast tributaries was undertaken for the 

 first time in connection with the Florida red-tide 

 studies. Thus, a few introductory remarks in 

 regard to the iniportance of iron in the growth 

 of phytoplankton seem to be appropriate. Iron 

 occurs in many different physical and chemical 

 forms in sea water (Ryther and Kramer, 1961). 

 It may be present in generally unstable organic 

 compounds that are slowly hydrolyzed in sea 

 water (Harvey, 1937). The quantity of iron in 

 true solution is extremely small because of the 

 insolubiUty of ferric hydroxide. 



The importance of u-on in the physiology of 

 phytoplankton has been stressed repeatedly 

 (Harvey, 1957; Sverdrup, Johnson, and Fleming, 

 1942). Algae are able to use particulate iron as 

 then- major source of this element through in- 

 gestion by the protoplasm of particles of ferric 

 hydroxide (Goldberg, 1952). Gran (1933) and 

 Menzel and Ryther (1961) demonstrated the 

 physiological dependence of phytoplankton on 

 u-on. Ryther and Kramer (1961) studied the 

 relative iron requirements of some coastal and 

 offshore planktonic algae. The consistency of their 

 results demonstrates the importance of iron in 

 determining the distribution of phytoplankton 

 in the sea. Iron is beneficial in the cultiu-e of 

 Gymnodinium breve (W. Wilson, personal com- 

 munication). Even though the physiological im- 

 portance of iron is known, no reliable techniques 

 exist for determining the form in which it is 

 available to phytoplankton (Harvey, 1937). 



The concentration of iron in the sea is very 

 small— from to 10 Mg. per Uter (Ryther and 

 Kramer, 1961). Quantities of this metal are much 

 higher in drainage waters than in adjacent seas 

 (Harvey, 1937). 



The individual concentrations of iron in our 

 study varied from 24.2 Mg. per Uter (at the sm-- 

 face in May at station 1, Hillsborough River) 

 to 1540.4 Mg. per liter (at the bottom in December 

 at station 5, Peace River). The mean values for 



FLORIDA'S WEST COAST TRIBUTARIES 



the entire period varied from 187.5 Mg. per liter 

 (at the surface of Little Manatee River) to 

 683.0 Mg. per hter (at the bottom of Myakka 

 River, station 6). Individual values were highest 

 at the fresh-water stations in the Myakka and 

 Peace Rivers and lowest in the Hillsborough 

 River (figs. 2, 7, and 9). In 83 percent of all 

 observations, iron values were from 50 to 599.0 ^ig. 

 per hter; in 63 percent the concentrations were 

 higher near the bottom than near the siu-face. 

 The monthly changes in iron were very irregular 

 at all stations (figs. 2-11) throughout the obser- 

 vation period. 



Concentrations of iron in the rivers were several 

 times liigher than those in the adjacent sea and 

 decUned progressively in the seaward direction 

 (table 1). River waters contribute iron to bay 

 waters, but tliis influence is negUgible offshore. 



The liighest concentrations of iron in the. major 

 rivers were in the area of Umestone formation 

 where the surface rock beneath the thin veneer 

 of Pleistocene sands was either Miocene or OUgo- 

 cene hmestone. Iron values were intermediate in 

 the major river (Myakka), which flows through 

 Miocene phosphatic clays, and the difference was 

 considerable between upriver values (station 5, 

 fig. 6) and downriver values (station 6, fig. 7). 

 This change can be directly attributed to the 

 change from Pleistocene to Miocene deposits 

 downstream. The lowest values were in the 

 Alafia and Peace Rivers which flow through 

 Pleistocene shelly sands of the Buckingham marl. 

 The limestone areas are highly soluble to car- 

 bonated rain waters and carry the iron into 

 solution \vith the CaCOa- As long as iron is in 

 the Fe+^ state, it moves readily; but when it is 

 oxidized, it is precipitated as insoluble Fe(0H)3. 

 An occasional association between the increased 

 rainfall and river discharge and liigher iron con- 

 centrations was probably due to the oxidation 

 of the iron in the limestone soils, and subsequent 

 fixation. In times of liigh rainfall, iron moves 

 into the rivers before oxidation can take place. 



The total quantity of iron contributed to Gulf 

 of Mexico waters by the rivers was estimated by 

 multiplying average concentrations by flow. Even 

 though monthly changes of iron concentrations 

 at individual stations lacked a trend, combined 

 data for all rivers showed that the contribution 

 of iron by the rivers to the Gulf was greatest 

 during maximum discharge (table 4). 



473 



