I. Pools and Fluxes of Nutrients in Florida Bay Sediments and Biological and Chemical Barriers to Fluxes at the Sediment-Water Interface

Larry Brand

University of Miami, RSMAS

Grant #NA67RJ0149

September 1, 2000

II. Abstract

This report is one of three reports on the research carried out in collaboration with Alina Szmant, Paul Carlson, and Laura Yarbro, in which the interaction of the biological and chemical processes in the sediments and water column of Florida Bay were examined.

III. Executive Summary

We hypothesized and have confirmed that indeed there is more microalgal biomass per area at the sediment surface than there is in the water column above in Florida Bay, generally 20 to 40 times as much. On top of the small-scale patchiness, there is a general shift toward higher benthic microalgal biomass in the west and lower levels in the east. This correlates with the much higher concentrations of P in western Florida Bay sediments. There is also a strong east-west trend in nutrient limitation of the benthic microalgae; those in the west are N limited and those in the east are P limited. This is in general agreement with the spatial pattern of both nutrient limitation in phytoplankton and water column inorganic N:total P ratios.

A comparison of turbidity data collected between 1973 and 1976 and between 1996 and 2000 indicate a dramatic increase in turbidity in Florida Bay. While some of the increase in turbidity is most likely the result of the massive seagrass dieoff in the 1980's in the central and western parts of the bay, much of it occurred well before the seagrass dieoff.

IV. Purpose

The overall goal of this research was to examine the hypothesis that benthic processes could be generating the algal blooms observed in Florida Bay. Of primary concern was the size of the benthic microalgal community compared to the phytoplankton community and whether or not benthic microalgae could be migrating or getting resuspended up into the water column to generate the observed algal blooms.

V. Approach

Our approach was to measure and examine the spatial and temporal distribution of benthic microalgae, water column phytoplankton, and water column turbidity; and examine their relationships. In addition, nutrient bioassays were conducted to determine the limiting nutrient for the benthic microalgal communities and the phytoplankton.

Water samples were taken 0.5 meters below the surface, using a small boat approximately monthly. Sediment cores were taken twice a year by SCUBA divers. The location of each of the samples was identified using a Global Positioning System. Temperature and salinity measurements were made with a YSI meter.

For water column chlorophyll measurements, three 100 ml replicate water samples were filtered (after adding 1 mg of MgCO₃) through GF/F glass fiber filters and the filters were frozen until extracted (within a few days). These filters were extracted for one hour with 10 ml of dimethyl sulfoxide and then with an added 15 ml of 90% acetone at 5^oC overnight and measured fluorometrically before and after acidification for the measurement of chlorophyll and phaeopigment concentrations (Burnison, 1980; Parsons et al., 1984). Fluorescence measurements were made with a Turner Designs 10-000R fluorometer equipped with an infrared-sensitive photomultiplier and calibrated using pure chlorophyll a.

Microalgal biomass in the sediments was estimated by measuring chlorophyll concentration using the method of Whitney and Darley (1979), which uses hexane-acetone partitioning to eliminate the chlorophyll degradation products found in sediments. Three replicate sediment cores down to 2 cm were frozen until ready for extraction. The sediments were extracted repeatedly with 90% acetone until pigment concentrations declined substantially in the sample, and the extracts mixed together. To eliminate degradation products, these extracts were then mixed with hexane and partitioned. The chlorophyll in the hexane fraction was then measured fluorometrically.

Turbidity was measured with a LaMotte 2020 Turbidimeter.

Nutrient limitation bioassays were conducted on whole water samples and "resuspended" sediment samples, using the methods of Brand et al. (1991). Either no nutrients (as a control), 250 mM NO₃; 25 mM PO₄; or 250 mM NO₃and 25 mM PO₄ (as a control) were added to 30 ml water samples and monitored daily with a Turner Designs 10-000R fluorometer for 1 to 2 months. To bioassay the sediments, a small amount of surface sediment surface was added to the overlying water to yield a turbidity of around 100 NTU to simulate sediment resuspension.

VI. Findings

We hypothesized and have confirmed that indeed there is more microalgal biomass per area at the sediment surface than there is in the water column above in Florida Bay, generally 20 to 40 times as much. Water column chlorophyll concentrations typically ranged from 0.5 to 10 mg/m² while benthic chlorophyll concentrations ranged from 24 to 120 mg/m².

A great deal of small scale patchiness characterizes the distribution of benthic chlorophyll in Florida Bay. Despite considerable variability, no obvious seasonal change is observed when comparing benthic microalgal biomass in winter (Fig. 1) and summer (Fig. 2). On top of the small scale patchiness, there is a general shift toward higher benthic microalgal biomass in the west and lower levels in the east (Fig. 3). This correlates with the much higher concentrations of P in western Florida Bay sediments observed by Yarbro and Carlson (1998). There is also a strong east-west trend in nutrient limitation of the benthic microalgae (Fig. 4); those in the west are N limited and those in the east are P limited. This is in general agreement with the spatial pattern of both nutrient limitation in phytoplankton (Fig. 5) and water column inorganic N:total P ratios (Fig. 6).

The amount of nutrients in the benthic microalgal biomass is similar to the total amount measured in the sediments. This suggests that benthic microalgae control the nutrient concentrations in the sediments and the flux of these nutrients in and out of the sediments. Therefore the most likely process by which the benthic microalgae could be generating the water column bloom is by actively migrating up into the water column or by being physically resuspended up into the water column. The lack of any spatial correlation between benthic microalgal biomass (Fig. 7) and phytoplankton biomass (Fig. 8) suggests that active migration is not a major factor, although the data cannot be considered as strong evidence. The overall correlation between benthic and water column chlorophyll is also poor (Fig. 9). Similarly, there is no obvious spatial correlation between benthic microalgal biomass (Fig. 7) and turbidity (Fig. 10), or between phytoplankton biomass (Fig. 8) and turbidity (Fig. 10). The overall correlation between water column chlorophyll and turbidity in east (Fig. 11), central (Fig. 12) and west (Fig. 13) Florida Bay is poor. Often high chlorophyll concentrations are associated with low turbidity, and high turbidity is often associated with low chlorophyll concentrations. This indicates that resuspension of benthic microalgae is not the dominant source of water column chlorophyll concentrations.

A comparison of turbidity data collected between 1973 and 1976 by T. Schmidt (Fig. 14) and our data between 1996 and 2000 (Fig. 10) indicate a dramatic increase in turbidity in Florida Bay. An examination of the average data in the 1970's indicates that the water was probably always somewhat turbid along the extreme northern coastline of the bay. The entire bay however was much less turbid at that time than today. During the rainy season, when wind speeds are usually low, turbidity levels are only around twice as high in the 1990's (Fig. 15) as in the 1970's (Fig. 16). During the dry season, however, when wind speeds are usually higher, turbidity levels are much higher than in the rainy season in most parts of the bay in both the 1970's (Fig. 17) and 1990's (Fig. 18). Dry season turbidities are also dramatically higher in the 1990's than in the 1970's. This is particularly true in eastern Florida Bay.

While some of the increase in turbidity is most likely the result of the massive seagrass dieoff in the 1980's in the central and western parts of the bay, much of it occurred well before the seagrass dieoff. Both human observations and satellite imagery indicate that the turbidity increased before the algal blooms or seagrass dieoff began. It appears that the large increase in turbidity may be primarily the result of increased runoff from the Everglades-agricultural system.

VII. Evaluation

Large cuts in the budget reduced the scope of our sampling and prevented the experimental work from being conducted, so only correlations could be made. Presentations of our results have been made at the Florida Bay Science Conferences and another presentation will be made at the Ocean Optics 2000 meeting.

Literature Cited

Burnison, B.K. 1980. Modified dimethyl sulfoxide (DMSO) extraction for chlorophyll analysis of phytoplankton. Can. J. Fish. Aq. Sci. 37: 729-733.

Parsons, T. R., Y. Maita and C. M. Lalli. 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. 173 pp., Pergamon Press.

Whitney, D.E. and W.M. Darley. 1979. A method for the determination of chlorophyll a in samples containing degradation products. Limnol. Oceanogr. 24: 183-186.

Yarbro, L.A. and P.R. Carlson. 1998. Seasonal and spatial variation of phosphorus, iron and sulfide in Florida Bay sediments. 1998 Florida Bay Science Conference, May, 1998, Miami, Florida.

Figure Legends

Figure 1. Average benthic chlorophyll concentrations in winter.

Figure 2. Average benthic chlorophyll concentrations in summer.

Figure 3. Frequency distribution of benthic chlorophyll concentrations in east, central and west Florida Bay.

Figure 4. Results of nutrient bioassay experiments conducted with resuspended sediments.

Figure 5. Results of water column nutrient bioassay experiments.

Figure 6. Average ratios of dissolved inorganic N to total P measured from 1991 to 1998. (calculated from SFWMD data files)

Figure 7. Average benthic chlorophyll concentrations.

Figure 8. Water column chlorophyll concentrations measured between February, 1996 and April, 2000.

Figure 9. Correlation between benthic chlorophyll concentrations and water column chlorophyll concentrations.

Figure 10. Average turbidity measured in 1996-2000.

Figure 11. Correlation between water column chlorophyll concentrations and turbidity in east Florida Bay.

Figure 12. Correlation between water column chlorophyll concentrations and turbidity in central Florida Bay.

Figure 13. Correlation between water column chlorophyll concentrations and turbidity in west Florida Bay.

Figure 14. Average turbidity measured in 1973-1976 (Tom Schmidt, unpublished data).

Figure 15. Average turbidity measured in summer, 1996-2000.

Figure 16. Average turbidity measured in summer, 1973-1976 (Tom Schmidt, unpublished data).

Figure 17. Average turbidity measured in winter, 1973-1976 (Tom Schmidt, unpublished data).

Figure 18. Average turbidity measured in winter, 1996-2000.