Investigators:	Randolph Chambers James W. Fourqurean
		Additional Project Information at:	--

Project Topic: Coastal & Estuary

Start Date: 10/1/1997
End Date: 10/1/1999

Project Description:
This study was completed to examine the coupling of biogeochemical processes between the sediment and water column in Florida Bay, with a specific goal of determining the potential impact of these processes on seagrass and algal production. Sediment samples from Florida Bay and the southwest Florida shelf were collected and analyzed to determine the concentrations of iron-sulfide minerals and selected forms of iron and phosphorus. We found that both iron and sulfur exhibited a general gradient of decreasing concentration from north to south in Florida Bay, but that phosphorus decreased from west to east across the bay. All three elements tended to occur in higher concentrations in the bay relative to sediments on the southwest Florida shelf. The predominant sulfide mineral in bay sediments is pyrite (up to 50 µmoles per gram dry weight of sediment), with concentrations of mackinawite typically an order of magnitude lower. A four-step extraction scheme for forms of iron and phosphorus determined that most sediment phosphorus in Florida Bay was associated with calcium carbonates and not with iron. A strong positive correlation between total iron and phosphorus was found for unvegetated sediments from the southwest Florida shelf, but not for sediments from Florida Bay. We found no evidence suggesting a phosphorus and/or iron source from a hypothesized Miocence sand channel. Further, it does not appear that reactive iron oxides are a major influence on the dynamics of phosphorus cycling throughout much of Florida Bay.

An index of iron availability, used to quantify the capacity of sediments to buffer sulfide production and act as a cap to phosphorus release, was smallest in the north-central portion of Florida Bay. This portion of the bay is where seagrass die-offs were most extensive a decade ago and where water column algal blooms are ongoing. Water column chlorophyll concentrations were negatively correlated with the index of iron availability, showing the strong coupling of sediment-water column processes: where iron availability in the sediments is low, there is more primary production in the water column (algae). Seagrass die-offs could either be a cause or a consequence of this sediment characteristic.

By stimulating authigenic sulfide mineral formation through the experimental addition of reactive iron to sediments, we demonstrated that iron-sulfide mineral formation in bay sediments was limited by the availability of reactive iron. Eight months after adding iron oxides to small plots near Rabbit Key Basin, both mackinawite and pyrite concentrations increased 10-cm into the sediment. Also, the amount of phosphorus sorbed to sediments increased dramatically in surface sediments, representing phosphorus retention in the sediments and/or removal from the water column. Iron oxides buffered sediment sulfide toxicity to seagrasses as documented by heavier stable sulfur isotopic signatures in leaves, and the seagrasses responded by producing more vigorous shoots. Because iron oxides also exacerbate nutrient limitation to seagrasses by sorbing phosphorus, standing crop did not increase. In these experiments we also measured much higher mineral sulfide concentrations in the winter relative to the summer, suggesting a dynamic seasonal component to mineral sulfide formation/dissolution in Florida Bay sediments.

Based on our results, we conclude that sulfide toxicity in sediments may contribute to seagrass die-offs, but that the plants are well-adapted to this carbonate environment where iron concentrations naturally are low and the buffering capacity is scant relative to other estuaries with terrigenous sediments. The major influence of sediment sulfide may be its reduction of reactive iron oxides, making iron unavailable to serve as a sorption site for reactive phosphorus. Phosphorus may be freer to migrate to the water column where it can stimulate algal growth. Although a large-scale addition of iron oxides to Florida Bay potentially could limit algal growth by sorbing water column phosphorus, the growth of sulfide-adapted seagrasses is capable of performing this task naturally. Our hypothesis that seagrass die-offs-whatever their cause-lead to a positive feedback of decreased system resistence to declining water quality is supported by the negative correlation between the sediment index of iron availability and water column chlorophyll concentrations. The observation of continuing recovery from the extensive seagrass die-off more than a decade ago, however, suggests the Florida Bay system is resilient to these sorts of perturbations and that multiple disturbances have been the rule-not the exception-in this sub-tropical ecosystem.