Roger W. Bachmann

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m−3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.

The restoration of Lake Apopka in relation to alternative stable states

Lake Apopka (Florida, USA) changed in 1947 from being a clear, macrophyte-dominated lake, used primarily for fishing, into a turbid algal lake with a poor fishery. The lake has resisted various efforts to reverse the change and restore the previous state. The restoration approach emphasizes the reduction in phosphorus inputs to reduce algal blooms and clear the water. We examined the question of whether a deep-lake approach with nutrient reductions is going to work on this large (area 124 km2) and shallow (mean depth 1.7 m) lake, or if techniques such as drawdowns or wind barriers developed for shallow lakes using the theory of alternative stable states are more applicable.The assumptions upon which the current restoration is based are not supported. The poor transparency is due more to resuspended sediments than plankton algae, so the current Secchi disk depth of 0.23 m is predicted to increase to 0.34 m with any reasonable reduction in algal levels. The failure of the macrophytes to become re-established probably is due more to unstable sediments than lack of light reaching the lake bed, and the marsh flow-way developed by the St Johns River Water Management District will be ineffective in removing particles from the lake. It would take more than 300 years to remove the fluid mud and more than 800 years to remove the rest of the low density sediments. We conclude that the loss of macrophytes in Lake Apopka is an example of a forward switch in the theory of alternative stable states, and that it will take more than a nutrient reduction program to bring about the reverse switch to a macrophyte state. We suggest an alternative approach using wave barriers to create refuges for plants, macroinvertebrates, and fish to restore Lake Apopkas largemouth bass fishery.

Use of an alternative method for monitoring total nitrogen concentrations in Florida lakes

We studied a recently described method for the determination of total nitrogen in natural waters involving sample oxidation with persulfate and subsequent determination of nitrate-nitrogen with second derivative spectroscopy and compared it to the USEPA approved method involving the sum of nitrate-nitrogen and Kjeldahl-nitrogen as measured with an automated analyzer. The overall objective was to determine if the two methods gave the same estimates of total nitrogen and if the detection limits, precision and accuracy of the new method were as good as those of the USEPA method. The new method was used to make measurements on replicated blanks, standards and lake water samples covering a range of concentrations. We also collaborated with certified laboratories to make comparative measurements on 5 standards and 21 lake water samples that were run by us with the new method and by them with the USEPA method. The new method had an instrument detection limit of 0.07 mg 1−1, and the standard deviation of 20 sets of lake water samples averaged 0.03 mg 1−1. The new method gave concentrations equivalent to those found with the USEPA method, was more sensitive, and had a higher degree of precision. We concluded that the new method is suitable as a substitute for the USEPA method. We also found that the addition of acid to lake water samples stored under refrigeration was not necessary to preserve them for later determinations of nitrate-nitrogen and total nitrogen and that freezing was an effective means of sample preservation for 90 days.

Volunteer Lake Monitoring: Testing the Reliability of Data Collected by the Florida LAKEWATCH Program

ABSTRACT Because the use of volunteer samplers is a very cost-effective way to collect large amounts of information on lakes over space and time, we studied the reliability of the protocols used by the Florida LAKEWATCH program. We found that chlorophyll extractions with hot ethanol gave values that were no different from those obtained with the standard method of grinding with acetone. In a comparative study of 125 lakes we found the data collected by volunteers were comparable to those collected by professionals. Mean Secchi disk depth, TP, TN, and chlorophyll values obtained by the citizens were strongly correlated (r > 0.99) to the mean values obtained by the professionals. To determine if freezing was a valid means of preserving water samples prior to analysis, we compared estimates of chlorophyll, TP, TN, pH, total alkalinity, and specific conductance obtained from fresh samples with estimates obtained from samples frozen up to 150 days. For most lakes there was little difference in chemical measurements made in samples preserved by freezing for different lengths of time, and various statistical tests indicated that freezing was a valid means of preserving lake water samples prior to analysis. Water quality data produced by volunteer samplers following the LAKEWATCH protocols were just as good as those from samples collected by professional biologists and handled using standard methods of sample preservation. The fact that volunteers can collect credible data means that lake management agencies could amplify their limited budgets by using volunteer monitoring, to sample more lakes and to sample them more frequently.

The Potential For Wave Disturbance in Shallow Florida Lakes

ABSTRACT We applied wave theory to calculate the extent and frequency that we would expect wave-driven surface water movements to disturb the sediments in 36 Florida lakes covering a broad range of surface areas and mean depths. The calculated per cent of the lakebed subject to wave disturbance at one time or another ranged from 6 to 100% and the per cent of the time 50% of the lakebed was disturbed ranged from 0 to 65%. The large Florida lakes, Apopka, Okeechobee, and Istokpoga showed high levels of calculated wave disturbance, which was consistent with the conclusions of previous investigations. Historic water level fluctuations in Lake Apopka were calculated to have major effects on wave disturbance in that lake. The dynamic ratio (the square root of lake surface area in square kilometers divided by the mean depth in meters) was significantly related to various measures of wave disturbance in our sample lakes. For lakes with ratio values above about 0.8 the entire lakebed was subject to wave disturbance at least some of the time. The dynamic ratio was also related to lake water quality. We found that increases in the dynamic ratio were significantly related to decreases in water quality as measured by total phosphorus, total nitrogen, chlorophyll, and Secchi disk depth. Calculations of wind disturbance by waves need to be modified in lakes with extensive beds of macrophytes, where water levels change and in periods where climatic fluctuations result in changes in wind regimes.

Trophic status of Iowa Lakes in relation to origin and glacial geology

Natural and artificial lakes in the most recently glaciated portions of Iowa have significantly greater total ion concentrations than those in other areas of the state. A similar distribution was found for total nitrogen concentrations. Lake origin seems of greater importance than location in determining trophic state. As a group, the artificial lakes have lower concentrations of total phosphorus, total chlorophyll, and greater Secchi disk transparency than do the natural lakes. This seems related to differences in the dynamics of the phosphorus cycles in these lake types.

Internal heterotrophy following the switch from macrophytes to algae in Lake Apopka, Florida

Measurements of photosynthesis and community respiration in Lake Apopka, Florida, U.S.A. indicate that this lake may be heterotrophic, and that the source of extra organic carbon is internal rather than external to the lake. This large and shallow lake (area 124 km2, mean depth 1.7 m) was dominated by macrophytes until hurricane-associated winds disrupted the plants in 1947, and the lake switched to a turbid, algal state. A layer of flocculent, organic sediments covers the lakebed to an average depth of 45 cm and winds regularly resuspend the upper portion into the water column. We used the diel oxygen curve method to estimate production and respiration and also reanalyzed the results of five past studies of production in the lake. The production measurements did not support the hypothesis that the flocculent layer represented excess algal production since 1947. Community respiration exceeded gross production on 60 out of 76 days sampled with statistically significant negative net production found in two of the three studies using the light and dark bottle oxygen method. External supplies of organic carbon are relatively small and are balanced by losses through the outlet. If the lake is heterotrophic, the excess respiration is most likely supported by the remains of macrophytes deposited in the sediments prior to the switch to an algal state. Similar sediment oxidation probably occurs in other shallow lakes that switch from the macrophyte to the algal state.

In situ nutrient enrichment experiments with periphyton in agricultural streams

Four agricultural streams were examined to determine if nitrogen or phosphorus was limiting the growth of the attached algal communities. Experiments were conducted in situ using nutrient-diffusing artificial substrata. Experiments conducted over a 2-year period demonstrated that the benthic algal communities in these streams were seldom limited by nitrogen or phosphorus. Nitrogen was, however, found limiting on two occasions when ambient nitrogen levels were reduced. These experiments were characterized by extended low flows and warm water temperatures. Large algal mats which proliferate during these periods may be responsible for reducing the available nitrogen to a limiting level. Major storm events are thought to interrupt the development of nitrogen limiting conditions by scouring the algae and increasing the nitrogen loading from the watershed. Water temperature was demonstrated as an important factor in controlling periphyton growth rates and may have influenced algal response to nutrient input. Ammonium additions often enhanced algal growth in the absence of nitrate stimulation.

Effects of climate variability on transparency and thermal structure in subtropical, monomictic Lake Annie, Florida.

Lake Annie is a small (37 ha), relatively deep (21 m) sinkhole lake on the Lake Wales Ridge (LWR) of central Florida with a long history of study, including monthly limnological monitoring since June, 1983. The record shows high variability in Secchi disc transparency, which ranged from < 1 to 15 m with a trend toward decreasing values over the latter decade of record. We examined available regional meteorological, groundwater and limnological data to determine the drivers and thermal consequences of variability in water transparency. While total nutrient concentrations and chlorophyll-a were highest during years of low transparency, stepwise regression showed that none of these had a significant effect on transparency after water color was taken into account. Repeated years of high precipitation between 1993-2005 caused an increase in water table height, increasing the transport of dissolved substances from the vegetated watershed into the lake. Groundwater stage explained 73 % of the interannual variability in water transparency. Transparency, in turn, explained 85 % of the interannual variability in the heat budget for the lake, which ranged from 1.8 × 10 8 to 4.1 x 10 8 Joules m -2 yr -1 , encompassing the range reported across Florida lakes. While surface water temperature was not affected by transparency, depths below 5 m warmed faster during the stratified period during years having a lower rate of light extinction. We show that an increase in precipitation of 20 cm per year reduces the depth of the summer euphotic zone and thermocline by 1.9 and 1.6 m, respectively, and causes a 1-month reduction in the duration of winter mixing in this monomictic lake. Because biota have been shown to respond to shifts in light and heat distribution of much smaller magnitude than exhibited here, our work suggests that subtle changes in precipitation linked to climate fluctuations may have significant physical as well as biotic consequences.

A Management Alternative for Lake Apopka

ABSTRACT We examine the idea that whenever high levels of planktonic algae impair a lake, nutrient control is the first step to be taken regardless of what other management actions might be contemplated. Our example is Lake Apopka, a 12,500 ha, shallow (1.7 m), polymictic lake in central Florida. Prior to 1947, the lake was dominated by macrophytes, was reputed to have clear waters, and had a national reputation for its largemouth bass fishery. Following a hurricane in 1947, the lake switched to a turbid, algal state and the bass fishery is all but gone. For over 30 years, it was either implied or stated directly that nutrient enrichment from anthropogenic activities, especially farms along the north shore, was to blame for this change and the lack of macrophyte recovery. The current management plan is to remove the farm nutrient supply by purchasing the farms under the theory that this will lower the total phosphorus concentration in the water and thus restore the lake. We have developed the most probable phosphorus budgets for the lake based on the studies of others and have determined that a fluid mud layer that is frequently resuspended by the wind will lead to high internal loading and slow the drop in phosphorus concentration. The equilibrium phosphorus concentration will lie between 52 and 88 mg·m−3, so the lake will remain in its eutrophic state. We propose an alternative management plan using artificial reefs that will focus on restoring the largemouth bass fishery in the immediate future. This idea is attractive because bass fishing was the dominant use of this lake in the past, it can be accomplished without waiting for a change in trophic state, and it can produce results in a relatively short period of time.