Post-drought Conditions Research Articles

Recovery of submerged plants from high water stress in a large subtropical lake in Florida, USA

The spatial and temporal dynamics of submerged plants were examined in a large subtropical lake in Florida, USA. The objective was to characterize succession of the community following a natural experiment in 2000–2001, when release of water from the lake, followed by a severe drought, reduced water levels by 2 m, alleviating stress of multiple years of high water. A systematic survey of shoreline transects was used to compare attributes of submerged plants under pre-drought versus post-drought conditions. Initially, the plants did not respond to lower water because shoreline areas had high turbidity from resuspended sediments and, later algal blooms. In June 2000, approximately 2 months after the water level was lowered, Chara (a macro-alga) rapidly expanded across the near-shore landscape. For over 1 year, this plant strongly dominated the submerged plant community, with just scattered individuals or isolated beds of vascular plants, including Potamogeton, Vallisneria, and Hydrilla. This included a period when the lake reached a record low elevation, where much of the habitat became dry, and then subsequently re-flooded in late summer 2001. However, in November 2001, Chara rapidly declined and vascular taxa ( Hydrilla and Potamogeton) became dominant. They subsequently increased their biomass and spatial extent, and the previous Chara dominance did not return. Just prior to the loss of Chara, a frontal system passed over the lake, with wind velocities in excess of 30 km h −1 for 3 days. Concentrations of solids in the water more than doubled and uprooted Chara was observed floating in the water. In this large, wind-driven lake, Chara may only be an ephemeral pioneer because, lacking roots, it is probably more sensitive to excessive wind-related stress (e.g. wave energy and scouring) than vascular plants.

Leaf fluctuating asymmetry of Holm oak in response to drought under contrasting climatic conditions

Fluctuating asymmetry (FA) can be defined as small, random deviations from bilateral symmetry in structures that, typically, are bilaterally symmetrical. In plants, FA has been used as a tool for monitoring levels of ecological stress. In this work, I analyse the FA response to drought in the Holm oak ( Quercus ilex) in SE Spain, to test whether FA increases or diminishes when climatic conditions are more stressful. Leaves from three different cohorts, corresponding to one drought year (1995) and two wet years (1996 and 1997) were collected at five sites contrasting in rainfall conditions, from very low to rather high rainfall. In two of the zones, two different morphs of leaves were collected. Furthermore, one of the sites was chosen close to a river, to examine the effect of water enhancement in the trees during drought and post-drought conditions. The results revealed no differences in response to drought in terms of FA between leaf morphs. However, while plants from rainy sites showed a decline in leaf FA from drought to wet years, the water-enhanced site and the dry sites showed scant or negligible differences between years. There was a significant decrease in FA from rainy sites to dry sites. These results suggest that (1) plants living in the more stressful sites are more symmetrical, (2) the more symmetrical plants respond less to yearly variations in drought stress, and (3) the response to yearly variations in drought depend on the climatic conditions in which a tree is living.

Pulsed breeding of long-legged wading birds and the importance of infrequent severe drought conditions in the Florida Everglades

At the scale of ecosystems and regions, numbers of nesting long-legged wading birds are often highly variable from year to year, and much of this variation is thought to reflect variation in production or availability of prey animals in wetlands. Based on observations during and following a severe drought in the Florida Everglades (1989–1992), we predicted that large nesting events would be more likely immediately following droughts than at other times. Using a 38-year history of wading bird nesting events in the Florida Everglades, we tested the hypothesis that “supranormal” annual nesting events (numbers of nests >1 standard deviation above the long-term mean) would occur more frequently during the period of up to two years after severe droughts (stages <1 standard deviation below the mean) than after non-drought years. Within this database, we identified 8 supranormal events and 8 severe droughts; 7 of the nesting events occurred immediately after a drought, and 7 of the droughts were followed by a supranormal nesting event. There was a highly significant association between the two types of events. Because many studies suggest that wading bird reproduction is food-limited, this result implies that post-drought conditions somehow result in exceptional productivity and/or availability of small fishes and macroinvertebrates. We propose two biological mechanisms for this pattern and suggest that rare, severe droughts in the Everglades are a forcing function for wading bird population cycles and large-scale movements through the action of pulsed productivity in the aquatic food web.

Dynamics of coarse riparian detritus in a Hawaiian stream ecosystem: a comparison of drought and post-drought conditions

AbstractInput, standing crop, and export of coarse (>4 mm2) particulate organic matter (CPOM) was measured in Kaiwiki Stream, a forested stream on the Island of Hawaii. The study was conducted during and after a severe drought associated with the 1997–1998 El Nino/Southern Oscillation event. During the drought, stream discharge was reduced by ∼70% and no spates occurred. Hawaiian streams draining windward mountain slopes normally experience >20 spates per year. During the drought, the mean rate of CPOM export was 8% of the mean input rate; following the drought, the export rate increased to 40% of input. Rates of marked leaf transport within the study area increased 30- to 50-fold following the drought, and rates of stem transport increased 10- to 30-fold. Lower rates of CPOM export and transport during the drought suggested that retention and potential availability to detritivores were increased compared to the post-drought period. Differences in CPOM standing crops during and after the drought were not ...

Microbiology and chemistry of acid lakes in Florida: I. Effects of drought and post-drought conditions

Short term changes in acid loading and dissolved organic carbon (DOC) content were studied in relation to water column bacteria of ten acid lakes on the Katharine Ordway Preserve, Florida. Five clear oligotrophic lakes and five dark dystrophic lakes were sampled during and after a drought period in July and September, 1985. Water column bacterial densities, light extinction, chlorophyll a, DOC, pH, dissolved oxygen, nutrients, and other chemical variables were measured. Significant positive correlations existed among DOC, chlorophyll a, pH, and water column bacterial densities during the drought period. There were no significant changes in water column bacterial densities or pH of clear lakes in the post-drought period, despite a 4.6 fold increase in acid loading from rainfall. A 3 fold increase of DOC, a decline in pH, and decreased bacterial densities in dark lakes suggested inhibition of bacteria by DOC and pH. A decrease in the relationship of DOC to bacterial numbers in all lakes was also noted. The correlations among DOC, chlorophyll a, and pH were no longer significant. Using data from both time periods significant polynomial regressions were observed between DOC and bacterial density and DOC and chlorophyll a. Maximum bacterial numbers occurred at 20 mg C 1−1 of DOC. Above this bacterial numbers decreased also suggesting an inhibitory effect of DOC. Because pH was lower after DOC had increased in the dark lakes, the increase in acid conditions may have enhanced this inhibitory effect. The short term effects of DOC on the dark-lake bacteria greatly exceeded the influence of acid loading on clear-lake bacteria.