Steven J. Miller

MOVEMENTS OF FLORIDA APPLE SNAILS IN RELATION TO WATER LEVELS AND DRYING EVENTS

Florida apple snails (Pomacea paludosa) apparently have only a limited tolerance to wetland drying events (although little direct evidence exists), but their populations routinely face dry downs under natural and managed water regimes. In this paper, we address speculation that apple snails respond to decreasing water levels and potential drying events by moving toward refugia that remain inundated. We monitored the movements of apple snails in central Florida, USA during drying events at the Blue Cypress Marsh (BC) and at Lake Kissimmee (LK). We monitored the weekly movements of 47 BC snails and 31 LK snails using radio-telemetry. Snails tended to stop moving when water depths were <10 cm; however, there was no apparent effect of water depth on distance traveled in water depths >10 cm. Snails moved along the greatest positive depth gradient (i.e., towards deeper water) when they encountered water depths between 10 and 20 cm. Snails tended to move toward shallower water in water depths ≥50 cm, suggesting that snails were avoiding deep water areas such as canals and sloughs. Of the 11 BC snails originally located in the area that eventually went dry, three (27%) were found in deep water refugia by the end of the study. Only one of the 31 LK snails escaped the drying event by moving to deeper water. Our results indicate that some snails may opportunistically escape drying events through movement. The tendency to move toward deeper water was statistically significant and indicates that this behavioral trait might enhance survival when the spatial extent of a dry down is limited. However, as water level falls below 10 cm, snails stop moving and become stranded. As the spatial extent of a dry down increases, we predict that the number of snails stranded would increase proportionally. Stranded Pomacea paludosa must contend with dry marsh conditions, possibly by aestivation. Little more than anecdotal information has been published on P. paludosa aestivation, but it is a common adaptation among other apple snails (Caenogastropoda: Ampullaridae).

Spatial ecology of the crayfishProcambarus alleni in a Florida wetland mosaic

We investigated patterns of differential habitat occupation by the crayfishProcambarus alleni, a numerically abundant and trophically important species in freshwater marsh systems of southern Florida. Crayfish were collected from emergent wet prairies and sloughs in marshes forming the headwaters of the St. Johns River, Florida between August 1992 and December 1993. In addition to differences in plant species composition, wet prairies had greater plant biomass and lower water depths than sloughs. Mean density and biomass of crayfish were significantly higher in the densely vegetated wet prairies (28 per m2; 26.9 g per m2) than in aquatic sloughs (3 per m2; 1.5 g per m2). Crayfish density increased with increasing plant biomass (i.e., habitat complexity) in wet prairies, whereas crayfish density decreased with increasing water levels (i.e., hydroperiod) within slough habitats. Recruitment occurred throughout the study, and the majority of crayfish collected were small in size (<1.0 g). Ovigerous females were rarely collected and were found only in wet prairies. Differences in relative risk of predation, food availability, or a combination of these factors are likely generating differences in habitat occupation by the crayfishP. alleni in this wetland habitat mosaic. Efforts to restore and manage freshwater marshes in southern Florida (e.g., Everglades, Lake Okeechobee, Kissimmee River, St. Johns River) would benefit by considering spatial aspects of the ecology of indicator taxa such as crayfish.

Spatial and temporal patterns of sediment and nutrient accumulation in shallow lakes of the Upper St. Johns River Basin, Florida

We used paleolimnological methods to investigate spatial and temporal patterns of bulk sediment and nutrient (C, N, P) accumulation in Lakes Hell ‘n’ Blazes (A = 154 ha, zmax = 240 cm), Sawgrass (A = 195 ha, zmax = 157 cm) and Washington (A = 1766 ha, zmax = 322 cm), in the Upper St. Johns River Basin, Florida. The study was designed to evaluate long-term changes in sedimentation and nutrient storage in the basin, and was one component of a larger project addressing flood control, wetland restoration, and water quality improvement. These three study lakes are wide, shallow waterbodies in the upper reaches of the St. Johns River channel. Sediment mapping indicates soft, organic deposits are distributed uniformly throughout Lakes Hell ‘n’ Blazes and Sawgrass. In contrast, much of Lake Washington is characterized by sandy bottom, and organic sediment is largely restricted to the north end of the lake. Lakes Hell ‘n’ Blazes and Sawgrass are effective sediment traps because dense submersed macrophytes and their associated epiphytes reduce flow velocity, intercept suspended particles, and utilize dissolved nutrients. Abundant Hydrilla, combined with short fetch, prevents resuspension and downstream transport of sediments. Larger Lake Washington is probably wind-mixed and resuspended organic sediments are redeposited to downstream sites. 210Pb-dated sediment cores show that organic sediment accumulation began in all three lakes before 1900, but that bulk sediment and nutrient accumulation rates have generally increased since then. The increases are probably attributable, in part, to anthropogenic activities including 1) hydrologic modifications that reduced flow rates in the channel, 2) discharge of nutrient-rich waters from urban, agricultural and ranching areas, and, 3) introduction and periodic herbicide treatment of the exotic macrophytes Eichhornia and Hydrilla.

Long-term effects of prescribed fire on Cladium jamaicense crantz and Typha domingensis pers. densities

The importance of fire to the maintenance of herbaceous plant communities in Florida wetland ecosystems is widely acknowledged. However, despite the acceptance of fire as a natural and necessary disturbance, ecosystem responses to fire in these systems are still poorly understood. Of particular concern is the effect of fire on the dynamics of plant communities dominated by Cladium jamaicense Crantz and Typha domingensis Pers. High nutrient levels, primarily phosphorus, and prolonged hydroperiods have been associated with Typha expansion into Cladium dominated communities. Recent studies suggest that fire is a disturbance that may play a facilitative role in this process. The objective of this study was to monitor the long-term effects of a single prescribed fire on Cladium and Typha densities in a freshwater marsh in Florida. Transects located at two burned sites and one unburned site were sampled prior to and annually for four years following a prescribed, lightning-season fire. There was a significant increase (P < 0.01) in Typha at both burn sites for two years after the fire. However, this increase was temporary since Typha density declined to pre-burn levels in the third and fourth years post-burn. Cladium density at the burned sites either increased or remained unchanged throughout the study period. When the control site unexpectedly burned in the fourth year of the study, density changes of Typha were similar to those observed at the original burn sites. Overall, we did not see any lasting changes in Cladium and Typha as a result of the fires, even though soil nutrient levels and hydroperiods were within levels documented to enhance Typha expansion.

Germination of sawgrass, Cladium jamaicense Crantz, under varying hydrologic conditions

The objective of this study was to determine the effects of three hydrologic treatments (saturated, shallowly inundated, and deeply inundated) on sawgrass (Cladium jamaicense Crantz) germination. In addition, germination of sawgrass was evaluated under two different sowing treatments. There was a significant difference (P < 0.05) in germination of sawgrass seeds between the three hydrologic treatments. However, there was no significant difference in germination between different sowing treatments. Germination was highest in saturated samples (average 30.2%), intermediate in shallowly inundated samples (average 16.3%), and lowest in deeply inundated samples (average 9.3%). Faster growth and higher survival of sawgrass seedlings occurred under saturated conditions than under inundated conditions. There was also a distinct seasonal germination pattern in all samples, showing a surge of germination in mid-October to early-November.

Contrasting Patterns of Habitat Use by Prawns and Crayfish in a Headwater Marsh of the St. Johns River, Florida

Abstract We compared seasonal patterns of habitat use by the prawn Palaemonetes paludosus and the crayfish Procambarus alleni in Blue Cypress Marsh Conservation Area, Florida. Prawn densities were similar to those found in other oligotrophic wetlands of southern Florida, whereas crayfish densities were much greater than reported previously for other wetlands in the area. Prawns and crayfish had strikingly different patterns of habitat use. Prawn density and biomass were similar in wet prairies and sloughs, whereas crayfish density and biomass were significantly higher in wet prairies. Within habitats, the abundance of prawns and crayfish generally increased with increasing structural complexity and the abundance of crayfish generally decreased with increasing water depth. Differences in risk of predation, frequency of agonistic encounters, food availability, and other factors likely contributed to observed patterns of habitat use. Because of differences in their ability to burrow and avoid concentration into dry-season refugia, prawns and crayfish responded very differently to seasonal variation in hydrologic conditions. Prawn densities were initially low (following a severe drought) and then increased during much of the study period, whereas crayfish densities were relatively stable throughout the study period. Overall, it appears that prawns are more responsive to antecedent hydrologic conditions and crayfish are more responsive to the availability of suitable habitats such as wet prairies.

Dormant Season Prescribed Fire as a Management Tool for the Control of Salix caroliniana Michx. in a Floodplain Marsh

Expansion of woody species into herbaceous wetlands is a serious concern in wetland management. Prescribed fire is often used as a tool to manage woody species, although many species resprout after fire making control problematic. In this study, we assessed the usefulness of repeated dormant season fires for controlling Salix caroliniana (Michx.) in a floodplain marsh in Florida. Salix is a common shrub in southeastern marshes that resprouts prolifically after fire. We compared stem basal area, stem density, and cover of Salix in three adjacent sites in a floodplain marsh in east central Florida. One site was burned once in February 1997, another site was burned in February 1997 and then again in March 1999 and one site was left unburned. At the unburned site, Salix stem basal area, stem density, and cover increased over the course of the study. In the two burned sites, the first fire destroyed large diameter stems and stimulated production of sprouts. As a result, stem basal area and cover decreased but stem density remained unchanged. The second fire caused a decline in stem density and a further decline in cover. Changes in understory species composition and cover could not be attributed to the fires. Our results suggest that dormant season fires are effective in reducing Salix cover and basal area, and that repeated fires have greater effects than a single fire.

Phytoplankton dynamics in a chain of subtropical blackwater lakes: the Upper St. Johns River, Florida, USA

Abstract Fisher, M.M., S.J. Miller, A.D. Chapman and L.W. Keenan. 2009. Phytoplankton dynamics in a chain of subtropical blackwater lakes: the Upper St. Johns River, Florida, USA. Lake Reserv. Manage. 25:73–86. Spatial and seasonal patterns in phytoplankton biovolume and community composition were examined for a chain of lakes in the Upper St. Johns River (USJR), Florida, USA. There was a general downstream trend in both increasing phytoplankton biovolume, and dominance of the algal community by cyanobacteria. Total algal biovolume increased from 0.7 × 106μ m3/ml in the headwaters lake to 5.6 × 106μ m3/ml downstream in Lake Winder. Cyanobacteria dominated the downstream lakes, accounting for approximately 50% of total algal biovolume, yet constituted only 2% of total biovolume in the headwaters lake. The diatom assemblage, as well as water quality data, suggests that these blackwater lakes are mesotrophic to eutrophic and neither nitrogen nor phosphorus limited growth. Fifteen months of cyanobacterial biovolume data were compared to water quality data to determine principal regulating factors. A regression model indicates that the major factors correlated with cyanobacteria in these lakes are temperature, total Kjeldahl nitrogen, water level, and color, with temperature alone accounting for 54% of the variability in cyanobacterial biovolume. This analysis demonstrates that multiple interacting factors need to be considered when attempting to explain spatiotemporal patterns in algal dynamics.

Responses of a willow (Salix caroliniana Michx.) community to roller-chopping.

Abstract Approximately eight hectares of shrub swamp wetland, primarily consisting of willow (Salix caroliniana Michx.), were roller-chopped to test the efficacy of this management technique in returning the plant community to one dominated by herbaceous species. Roller-chopping caused severe disturbance to the treatment area. Few willows were left standing and a majority of trees were uprooted entirely. Three months following chopping, limited re-sprouting of willow was observed. However, by the following March, no live willows were found in the treatment area. Willow remained absent for three years post-treatment. Hydrologic conditions may have influenced willow mortality through drought-induced stress and subsequent inundation in the treatment area. In contrast, these hydrologic conditions did not appear to affect willows in the control area. Following the elimination of willow, herbaceous plant species re-colonized the treatment areas, increasing in richness and abundance. Free-floating plants became the dominant species in the newly opened areas. Gradually, free-floaters became less important while rooted emergents, such as graminoids and forbs, became more abundant. This study indicates that, under dry conditions followed by flooding, roller-chopping can be an effective method of willow control. However, utilization of heavy equipment may alter topography, compact soil, and affect drainage patterns. Therefore, managers should exercise caution when applying this technique.

An Inexpensive, Low-Maintenance, Precision Amplifier, Dissolved Oxygen Sensor, and Miniature Data Logger System Designed for Long-Term Deployment in Riverine Systems

Abstract The dissolved oxygen (DO) concentration is a dynamic water quality variable that is critical to fisheries. To obtain simultaneous DO data at 15-min intervals over long periods (i.e., months) at a number of points along a river, a simple, inexpensive (<US