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Influence of habitat and other factors on Largemouth Bass abundance in Lake Okeechobee, Florida
Abstract
Objective
Submerged aquatic vegetation (SAV) is an important part of shallow lake ecosystems. It provides protective cover and foraging habitat for age‐0 and juvenile Largemouth Bass Micropterus salmoides and other small fish, creates structure and habitat for macroinvertebrates and other fauna, and can influence the physical environment by stabilizing sediments and reducing water column turbidity.
Methods
For this study, we reviewed annual abundance data for Largemouth Bass collected via electrofishing from 2002 to 2019, along with environmental covariates that included SAV cover (habitat) and turbidity.
Result
All age‐groups of Largemouth Bass were positively correlated with vascular SAV cover. Age‐0 Largemouth Bass were most abundant when SAV cover exceeded approximately 24% (9400 ha) of the littoral and nearshore area, while the juvenile and all‐ages groups were most abundant when SAV cover exceeded 20% and 12% of the area, respectively. Following major storm events, water depth and turbidity increased, SAV cover was reduced to <5% of the littoral and nearshore area, and the abundance of age‐0 Largemouth Bass was reduced more than 90%.
Conclusion
Once removed from the landscape and exposed to turbid conditions, vascular SAV will reestablish only if water clarity increases and light is sufficient to support plant growth. Lowering water levels for an extended period has been shown to reduce nearshore turbidity and jumpstart the recovery of SAV in Lake Okeechobee and other shallow lakes globally. Without adequate SAV habitat, recruitment of age‐0 and juvenile Largemouth Bass will likely be suppressed. Maintaining a minimum coverage of vascular SAV (20–35%) in the littoral and nearshore area should increase the abundance of age‐0 and juvenile Largemouth Bass and provide beneficial habitat for adult Largemouth Bass and other lake fauna.
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Submerged aquatic vegetation (SAV) is declining worldwide, leading to subsequent reductions in the ecological functions associated with SAV in shallow aquatic ecosystems, including providing habitat for fishes. Extensive restoration efforts are required to reverse this trend, but studies focusing on aquatic vegetation have been uncommon in recent years relative to other primary producers. Evaluations of the most beneficial SAV species and characteristics for fishes are especially rare. Because of the potentially complex and inconsistent responses of fish to different management actions, further research is necessary to evaluate the species-specific and community-level effects of SAV to inform restoration decision-making. To examine what SAV characteristics increase fish habitat use in a turbid-algal lake undergoing restoration, we sampled 29 areas around Lake Apopka, Florida (USA) with fyke nets and trotlines. We examined the impact of eight environmental variables on fish abundance, biomass, community structure, and predation potential. For each approximated 0.6 m ² increase in SAV patch size, total fish biomass catch increased 6.5 g hr ⁻¹ . Fish community composition based on abundance also changed with an increase in SAV patch size. The number of bait items missing from trotlines, a measure of predation potential, was most affected by water temperature, wind speed, and time of day, but not by the SAV variables tested. These results expand existing knowledge of fish habitat use of SAV and will inform future management efforts to conserve and restore fish communities by focusing on specific SAV characteristics such as patch size.
Submerged aquatic vegetation (SAV) has well-documented effects on water clarity. SAV beds can slow water movement and reduce bed shear stress, promoting sedimentation and reducing suspension. However, estuaries have multiple controls on turbidity that make it difficult to determine the effect of SAV on water clarity. In this study, we investigated the effect of primarily invasive SAV expansion on a concomitant decline in turbidity in the Sacramento-San Joaquin River Delta. The objective of this study was to separate the effects of decreasing sediment supply from the watershed from increasing SAV cover to determine the effect of SAV on the declining turbidity trend. SAV cover was determined by airborne hyperspectral remote sensing and turbidity data from long-term monitoring records. The turbidity trends were corrected for the declining sediment supply using suspended-sediment concentration data from a station immediately upstream of the Delta. We found a significant negative trend in turbidity from 1975 to 2008, and when we removed the sediment supply signal from the trend it was still significant and negative, indicating that a factor other than sediment supply was responsible for part of the turbidity decline. Turbidity monitoring stations with high rates of SAV expansion had steeper and more significant turbidity trends than those with low SAV cover. Our findings suggest that SAV is an important (but not sole) factor in the turbidity decline, and we estimate that 21–70 % of the total declining turbidity trend is due to SAV expansion.
Previous laboratory studies have demonstrated that turbidity alters prey selection by Largemouth Bass Micropterus salmoides , but this has not been tested in the field. Laboratory studies have also suggested that daily rates of consumption by Largemouth Bass may decline as turbidity increases and that the rate of decline may differ among prey types, possibly providing a mechanism that drives the observed prey selection patterns. We conducted laboratory trials to measure daily consumption rates for Largemouth Bass when foraging on Bluegills Lepomis macrochirus and Gizzard Shad Dorosoma cepedianum ; we also collected field data in three reservoir systems (Boomer, Sooner, and Guthrie lakes, Oklahoma) during 2 years to evaluate Largemouth Bass prey selection and consumption rates over a range of turbidity levels. In the laboratory trials, Largemouth Bass daily consumption rates declined with increased turbidity, but the decline was similar between the two prey types. We detected no correlations between turbidity and consumption rate (percentage of empty stomachs or mean weight of prey in stomach contents) in the field. Field‐measured prey selection was highly variable among reservoirs and between sampling years, yet all but one reservoir ×× year combination had significant correlations between prey selection and turbidity, which generally followed the predictions of previous laboratory results (greater use of fish prey [especially centrarchids] and reduced use of crayfish at high levels of turbidity). However, the use of crayfish prey under low‐turbidity conditions was higher than that expected based on previous laboratory trials. Further research is needed to determine which factors regulate the effects of increased turbidity on predator foraging return and to evaluate the food web implications of turbidity‐related changes in the diets of top predators.
Received December 3, 2014; accepted June 25, 2015
oung-of-the-yealra rgemouthb assM icropteruss almoidesin Lake Opinicon,O ntario,d isplayed
a divergence in size over their first summer. The range in total body lengths was 11 mm when
year-0 fish were first captured on June 22-25, and this range increased to 26 mm by September
8-12. The size divergence was not due to a protracted or disrupted spawning period and, instead,
appearedt o be underlain by differencesin diet and prey availability. Diets of small and largey ear-0
largemouth bass (less than and greater than the cohort mean length, respectively) differed progressively
over the summer. Large year-0 fish maintained a diverse diet of which fish prey always
constituted 25-30% by volume. In contrast, small year-0 largemouth bass shifted from a diet
comprisedo f zooplankton,a quatici nsectsa, nd larval fishi n Julyt o oned ominatedb y zooplankton,
and including no fish, in September. The diet shift of the small individuals occurred when fish
prey became too large to handle. However, some forage fish were always within the manageable
size range for large year-0 largemouth bass and, consequently, large individuals were able to
maintain fish in their diet. Analysis of the gross energetic value of the diets showed that larger
year-0 individuals obtained a higher energy content in their diet, relative to the small individuals.
Littoral zones of lentic systems are important nursery habitats for age-0 fish as they provide foraging habitat and refuge from predation. While local habitat characteristics in a lake's littoral zone can affect abundances, mean lengths, and trophic interactions of age-0 fish, lake-scale characteristics may also influence these factors. Given that local and lake-scale characteristics are probably related, it may be difficult to interpret perceived associations between habitat characteristics and fish population metrics, as well as elucidate consequences of anthropogenic alterations of habitat. We used classification and regression tree (CART) analysis to evaluate relationships between local and lake-scale habitat characteristics, abundances, and mean lengths of age-0 Largemouth Bass Micropterus salmoides and Bluegills Lepomis macrochirus collected from glacial lakes in northern Indiana during 2009 and 2010. We also evaluated habitat influences on stomach contents of age-0 Largemouth Bass. Our results suggest there are complex, multiscale interactions among habitat characteristics and population and diet metrics of age-0 fish. Site-specific abundances of both fishes were most strongly associated with local-scale characteristics (e.g., high densities of Bluegills at sites with dense submerged vegetation), while mean lengths were associated with lake-scale characteristics (e.g., greater mean length of age-0 Bluegills and Largemouth Bass in small, shallow lakes). Diet composition of age-0 Largemouth Bass was strongly related to individual fish length, though local (e.g., percent coverage by vegetation) and lake-scale (e.g., lake depth) habitat characteristics also affected the type of prey consumed. We suggest that managers consider both local and lake-scale features when undertaking habitat improvement activities, selecting stocking locations, or otherwise jointly managing habitat and age-0 lentic fish populations.
We performed whole-lake manipulations of fish populations to test the hypothesis that higher trophic levels regulate zooplankton and phytoplankton community structure, biomass, and primary productivity. The study involved three lakes and spanned 2 yr. Results demonstrated hierarchical control of primary production by abiotic factors and a trophic cascade involving fish predation. In Paul Lake, the reference lake, productivity varied from year to year, illustrating the effects of climatic factors and the natural dynamics of unmanipulated food web interactions. In Tuesday Lake, piscivore addition and planktivore reduction caused an increase in zooplankton biomass, a compositional shift from a copepod/rotifer assemblage to a cladoceran assemblage, a reduction in algal biomass, and a continuous reduction in primary productivity. In Peter Lake, piscivore reduction and planktivore addition decreased zooplanktivory, because potential planktivores remained in littoral refugia to escape from remaining piscivores. Both zooplankton biomass and the dominance of large cladocerans increased. Algal biomass and primary production increased because of increased concentrations of gelatinous colonial green algae. Food web effects and abiotic factors were equally potent regulators of primary production in these experiments. Some of the unexplained variance in primary productivity of the world's lakes may be attributed to variability in fish populations and its effects on lower trophic levels.
Sport fish from Normandy Reservoir, Tennessee, were sampled for more than 6 years with a variety of gears targeting different life stages. Normandy Reservoir experienced different hydrologic regimes over the sampling period that we roughly grouped into dry years (1992 and 1995), intermediate years (1993 and 1997), and wet years (1994 and 1996). Year-class strength of largemouth bass Micropterus salmoides was fixed each year by late summer or early fall. Catch of age-1 largemouth bass in spring electrofishing samples was directly related to the number of days the reservoir was at or over full pool when the fish were age 0. Largemouth bass produced in a wet year and intermediate year were more than twice as abundant at age 3 than fish produced in two dry years. Recruitment of spotted bass M. punctulatus could not be linked to reservoir hydrology. Crappies Pomoxis spp., white bass Morone chrysops, and saugeyes (walleye Stizostedion vitreum × sauger S. canadense) produced poor year-classes in dry years and strong year-classes in wet years. The responses of these latter three taxa in intermediate years were variable, although they were more characteristic of dry-year responses than wet-year responses. Recruitment of crappies, white bass, and saugeyes was positively related to mean daily discharge of the reservoir in the prespawn period (1 January to 31 March) each year. Recruitment of largemouth bass was dependent on high water during the spring and summer when fish were age 0. Water-level fluctuation in this Tennessee reservoir played a pivotal role in regulating year-class strength of most sport fish species. Attempts to enhance year-class strength of fishes in tributary storage impoundments should focus on altering the hydrology of systems.
Effects of sampling time (day or night and fall or spring), target fish density, water clarity, water temperature, water conductivity, and lake morphometry on electrofishing catch per effort (CPUE) of largemouth bass Micropterus salmoides 200 mm total length (TL) and longer were determined. Electrofishing catch per hour (CPH) and catch per kilometer (CPK) were also compared to determine if each expression provided similar trends in CPUE. Correlations between day CPH and day CPK (r = 0.99; P < 0.0001) and night CPH and night CPK (r = 0.97; P < 0.0001) suggested that both measures provided similar trends in CPUE. Night CPH significantly exceeded day CPH, and spring CPH significantly exceeded fall CPH. Catchability (q) decreased with increasing density; therefore, CPH increased nonlinearly with density. Day CPH in fall decreased with increasing Secchi depth and water temperature but was unrelated to largemouth bass density. Day CPH in spring decreased with increasing Secchi depth and water temperature and increased with increasing density of largemouth bass and water conductivity. Night CPH in fall increased with increasing density and decreased with decreasing water conductivity, and night CPH in spring increased with increasing density and decreasing percent littoral area (percent of lake with depth less than 4.6 m) among lakes. These variables explained 44% of day CPH in fall, 75% of day CPH in spring, 28% of night CPH in fall, and 59% of night CPH in spring. Effects of density on q must be determined and environmental conditions must be similar before CPUE can be a useful index of largemouth bass density.
Patterns of wave amplitudes, linear flow velocity, turbidity, sedimentation and periphyton accumulation were quantified across two macrophyte beds of contrasting architecture and adjacent unvegetated sediments, in Lake Veluwe, The Netherlands. Within both a Chara aspera- and an Alisma gramineum-dominated bed, wave amplitudes were progressively reduced. Vertical profiles of linear velocity in the two vegetations were significantly different from each other and from profiles measured outside the vegetation. Compared to unvegetated profiles, flow over the Chara-bed was more rapid in the upper 20 cm of the water but then dropped rapidly. In Alisma, maximum velocity at the surface was already lower than outside the vegetation. The denser Alisma-bed had a higher biomass (125 +/- 15 g AFDW m(-2), mean +/- standard error) than the Chara-bed (28 +/- 1), it filled the whole water column, and rapidly reduced light attenuation (K-d) from 6.4 +/- 1.4 outside to 2.3 +/- 0.3 m(-1) within the vegetation. Also in the Chara-bed, light attenuation and sedimentation rates were significantly reduced (light attenuation: 1.2 +/- 0.1 versus 2.7 +/- 0.1 m(-1); sedimentation: 2.6 +/- 0.3 versus 7.7 +/- 0.5 mg dry weight cm(-2) d(-1), averaged over the growing season). The dense Chara vegetation of 10-20 cm height effectively lifted the resuspensive turbulent shear stress above the sediment surface, thereby trapping settled material. [KEYWORDS: wave height; water flow; Chara; Potamogeton; Alisma; Lake Veluwe; periphyton Lakes; seagrass; dynamics; decline; communities; biomass]
Turbidity in aquatic systems can change rapidly, affecting the visual ability of predators. Increased turbidity is known to reduce the reactive distance and foraging success of some planktivores and insectivores, leading to decreased growth rates. However, little is known about the effects of turbidity on prey selection by piscivores. We examined the interactions between largemouth bass Micropterus salmoides and their prey in 1.8-m-diameter tanks (58 cm deep) at four turbidity levels (0, 5, 10, and 40 nephelometric turbidity units [NTU]). Prey selectivity was significantly affected by turbidity. At lower turbidity levels (0 and 5 NTU), largemouth bass consumed mostly gizzard shad Dorosoma cepedianum and bluegills Leponis macrochirus and had negative selectivity for northern crayfish Orconectes virilis. At 10 NTU, all three prey types had similar selectivity, presumably because the largemouth bass had more difficulty in capturing rapidly moving fish prey as their reactive distances decreased. At 40 NTU, the overall foraging rate was much lower and bluegills were selected significantly more often than the other prey types. Low light levels at the bottom of the tanks combined with reductions in visual clarity front clay sediments probably made it difficult for largemouth bass to feed effectively on virile crayfish Lit higher turbidities. Our results suggest that trophic interactions may be altered as turbidity levels change.
Water turbidity has the capacity to influence fish foraging success and behavior. The largemouth bass Micropterus salmoides is a popular sport fish in the southeastern United States that is primarily a visual predator. The high turbidity in many systems can be attributed to sediment loading from agricultural lands, and it can reduce the availability of light in the water column and have direct impacts on largemouth bass foraging success. We investigated the effect of different turbidity levels (0, 10, 25, 50, 100, and 250 nephelometric turbidity units [NTU]) on largemouth bass foraging in aquaria by testing the hypothesis that turbidity has no effect on the time required to locate tethered prey (fathead minnow Pimephales promelas and golden shiner Notemigonus crysoleucas) or ultimate capture success. The percentages of prey captured that were derived from aggregated data in multiple trials at the different treatment levels differed significantly. One-hundred percent of the largemouth bass in the 0-NTU treatments captured the prey. Conversely, only 15% of the largemouth bass in the 250-NTU treatments captured the prey throughout all trials. The average time taken to capture the prey also was significantly different between treatment combinations, with time to interaction increasing as turbidity increased. The results from this study suggest that greater turbidity levels reduce the ability of largemouth bass to capture prey and increase the time taken to locate and interact with prey. Thus, turbidity may impact individual fitness and management strategies.Received April 1, 2011; accepted June 20, 2011
Young-of-the-year largemouth bass Micropterus salmoides in Lake Opinicon, Ontario, displayed a divergence in size over their first summer. The range in total body lengths was 11 mm when year-0 fish were first captured on June 22–25, and this range increased to 26 mm by September 8–12. The size divergence was not due to a protracted or disrupted spawning period and, instead, appeared to be underlain by differences in diet and prey availability. Diets of small and large year-0 largemouth bass (less than and greater than the cohort mean length, respectively) differed progressively over the summer. Large year-0 fish maintained a diverse diet of which fish prey always constituted 25–30% by volume. In contrast, small year-0 largemouth bass shifted from a diet comprised of zooplankton, aquatic insects, and larval fish in July to one dominated by zooplankton, and including no fish, in September. The diet shift of the small individuals occurred when fish prey became too large to handle. However, some forage fish were always within the manageable size range for large year-0 largemouth bass and, consequently, large individuals were able to maintain fish in their diet. Analysis of the gross energetic value of the diets showed that larger year-0 individuals obtained a higher energy content in their diet, relative to the small individuals.Received March 22, 1984 Accepted December 17, 1984
In a series of in situ enclosure experiments with larval bluegill (Lepomis macrochirus), we demonstrate that turbidity from suspended sediments reduces bluegill consumption of crustacean zooplankton, primarily cyclopoid copepods and copepod nauplii. However, this reduction occurred only when light intensity in parts of enclosures fell below a threshold, estimated at < 450 lx. Following recent studies demonstrating copepod diel vertical migration in response to predators, it appears that copepods in our experiments used low-light strata as a refuge. Without this apparent refuge present, larval bluegill consumption increased with increasing turbidity, but prey were smaller on average. Thus, prey biomass consumed by larval bluegill did not differ with turbidity in high-light conditions. We postulate that the shift to smaller prey across taxa at higher turbidity, when light intensity exceeded 450 lx, derives from increased prey–background contrast. In low-light conditions, larval bluegill consumed larger, but fewer, zooplankton with increasing turbidity, resulting in lower prey biomass consumed. Thus, we demonstrate the field conditions causing negative turbidity effects on larval fish foraging success, and thus growth and recruitment.
The aim of this paper was to identify the relationship between the degree of macrophyte cover on the lake bottom and the abundance and species richness of larva and juvenile stage assemblages. The analysis of the material verified the hypothesis that as the surface area of macrophyte cover increases so does the abundance, species richness, and diversity of fish aggregations. The investigations were conducted in the heated Lake Licheñskie in 2000-2003 during the period when underwater vegetation is at the height of development (June - September). The spatial differentiation of macrophyte occurrence had a significant impact on the occurrence and differentiation of the fry assemblages. It was confirmed that along with an increase in the macrophyte cover surface area there was also an increase in the species richness and diversity of juvenile fish assemblages. The locations that were most densely overgrown with vegetation had the highest fish biomass and individual weight. In littoral areas without macrophytes, abundant single-species concentrations of fish occurred. The bottom macrophyte cover did not have an impact on the numbers of fish. *
To determine if the density, biomass, species density (number of fish species per square metre), and species composition of juvenile and forage fish (fish ≤ 100 mm SL) differed among macrophyte types, we sampled fishes monthly from beds of Hydrilla verticillata, Panicum hemitomon, and Potamogeton illinoensis in two areas of Lake Okeechobee, Florida, from December 1989 to December 1990. Fish density and biomass differed significantly among the three macrophytes despite our design having relatively low statistical power to detect such differences. Densities and biomass were highest in H. verticillata (196 fish∙m−2, 30 g∙m−2), intermediate in P. hemitomon (60 fish∙m−2, 12 g∙m−2), and lowest in P. illinoensis (31 fish∙m−2, 6 g∙m−2). Trends among macrophytes were consistent throughout the year despite significant variation among months. Fish species density did not vary significantly among macrophyte types. However, the relative abundances of fishes differed significantly among macrophyte types. Patterns in fish abundance and relative species composition led us to infer that beds of different macrophyte types constitute distinct microhabitats for fish and structure the littoral zone of Lake Okeechobee into a mosaic landscape.
We evaluated the effects of stem density on the attraction of bluegills Lepomis macrochirus and largemouth bass Micropterus salmoides to mats of artificial vegetation in a 5-ha impoundment in southwestern Ohio. We also used field enclosures to examine the effects of artificial vegetation density on growth of age-0 blue gills and on predation risk of age-0 bluegills (< 50 mm total length, TL) from largemouth bass. The densities used were 0 (zero), 400 (low), 961 (medium), and 3,844 (high) stems/m2. Underwater surveys of mats indicated significant differences in numbers of age-0 bluegills among vegetation densities; high-density mats always attracted significantly more age-0 bluegills than other densities. Numbers of larger bluegills (>50 mm TL) were similar among vegetation densities, whereas largemouth bass were more abundant near high-density mats. Lengths of age-0 bluegills did not differ significantly among stem densities, suggesting that selection of cover density did not depend on fish size. In addition, growth of age-0 bluegills over 117 din enclosures did not differ significantly with respect to stem density. In field enclosures, high-density mats significantly reduced predation risk for age-0 bluegills relative to low- and zero-density mats; predation risk did not differ significantly between high- and medium density mats. Laboratory experiments indicated no differences in predation rate between zero- and low-density treatments, but predation rates were significantly lower in medium and high stem densities than in low and zero densities, and high-density mats offered significantly greater protection than medium-density mats. This study, together with information from other studies, provides evidence that age-0 bluegills need not sacrifice growth for safety when they select high vegetation density under natural conditions.
This report describes a proposed new technology area in the Aquatic Plant Control Research Program on fish/plant interactions. A plan of study (POS) is presented based on a comprehensive literature review of relationships between fish and aquatic plants, and input from two workshops held with State, Federal, and university researchers. The POS discusses the need for future research according to deficiencies in the literature and serves as a basis for prioritizing and designing studies on fish/plant interactions in different geographic regions of the United States. The purpose of the first two work units (Classification of Aquatic Plant Habitats for Fish; and Fish Distribution, Diversity, and Abundance in Vegetated Habitats) is to describe species composition and abundance of fishes associated with specific aquatic plant habitats. Once the habitat classification is completed, studies will be initiated to evaluate why fishes use specific plant habitats. This question addresses functional use of habitats by fishes and will include work units on reproduction, feeding, and growth. The functional value of plants to fishes is an essential element in understanding the complex interactions of biotic and abiotic factors that regulate population maintenance and community structure of fishes. The last work unit on management strategies will interrelate the results of the descriptive and functional studies to determine how aquatic plants should be managed for fish. Demonstration sites will be selected, and different management strategies will be applied.
In order to determine the influence of lentic habitats and macrophyte stands on the plankton development and seston flux, an investigation of zooplankton was carried out in the karst Jankovac flow-through system (Papuk Nature Park, Croatia). The system was characterized by low abundance (1-116 ind.L-1) and high diversity of identified zooplankton. Eighty-six taxa were recorded, comprising 57 rotifers, 15 cladocerans, 8 copepods and 6 members of other groups of organisms. The spatial oscillations of environmental parameters and biocoenosis assemblage revealed statistically significant differences between lotic and lentic habitats, as well as between vegetated and non-vegetated stations. These differences mainly respond to higher concentration of food resources and zooplankton/zooseston abundance and biomass in lentic, especially vegetated, habitats. This is also proved by results of principal component analysis (PCA), which suggested that the main drivers of development of the planktonic community were the food resources and the avoidance of flow velocity. Accordingly, shoreline areas with submerged macrophyte stands of Hippuris vulgaris L. were the most productive parts, represented by highest zooplankton abundance, biomass and biodiversity. Flow velocity significantly affected crustaceans assemblage, so that higher abundances of the larger cladocerans and copepods were achieved in vegetated stations with low flow velocity, while rotifers showed to be rheotolerance organisms. On the other hand, the longitudinal discontinuum of the stream channel by two man-made reservoirs could offer new habitats to enrich seston with organic particles and bioseston. The results of our study pronounce the need for further monitoring of this hydrosystem, especially considering biodiversity and microhabitats conservation.
Models for fish capture efficiency (catchability) using a boat-mounted AC electrofishing sampling protocol were estimated for warmwater fishes in Illinois lakes through a calibration process. Catchabilities were determined for 37 inshore zones in blocked enclosures or ponds and 5 large water bodies during fall or spring. The abundances of vulnerable fish populations were determined by census following draining or by treatment using rotenone or primacord of known catchability. Inshore catchabilities, based on a zone 0-13 m from shore, were strongly dependent on fish length (as a unimodal function), fish taxa, mean depth, and surface macrophyte cover. Under average environmental conditions, maximum catchabilities by taxon ranged from 0.0018 to 0.14 and ranked (highest to lowest) as follows: largemouth bass Micropterus salmoides, common carp Cyprinus carpio, crappies Pomoxis spp. in spring, shad Dorosoma spp., bluegill Lepomis macrochirus, green sunfish L. cyanellus, crappies in fall, freshwater drum Aplodinotus grunniens, suckers (Catostomidae), and catfish (Ictaluridae). Catchabilities for common carp and shad were significantly lower in large water bodies, indicating that significant numbers were outside the inshore zone. However, the results for other species, including large samples of largemouth bass and bluegills, indicated that populations in similar whole lakes could be estimated from the predicted inshore electrofishing catchabilities in fall or spring. Strong biases in relative population density when inferred by catch per unit effort were demonstrated under differing environmental conditions. Also, estimates of ratios related to fish size, such as mortality rates, were seriously biased when based on the size structure of uncorrected catches. Therefore, catchability models are considered essential for assessing the absolute and relative attributes of fish populations across water bodies.
Water-level fluctuations are among the major driving forces for shallow lake ecosystems. In the low-lying parts of the Netherlands, the water-level regime of lakes is strictly regulated. This is needed for reducing risks of flooding and economic purposes, including maximum agricultural benefit. The fixation of water levels has had a severe impact on the functioning of (semi-)aquatic ecosystems. We review the benefits of natural water-level fluctuations, considering the impacts on nutrient inputs, nutrient concentrations, phytoplankton development and turbidity. In particular, the mediating role of submersed and emergent vegetation and filter feeders is addressed. The present government policy, to allow more space for water, presents a major challenge for combining flood prevention measures and ecological restoration. Restoration of natural water-level regimes, which is likely to lead to enhancement of water quality and biodiversity, may occur in two ways: (1) expanding the critical limits between which the water level is allowed to fluctuate annually, and/or (2) incidental recessions of the water level. It is stressed that ecologically-based water-level regimes should be incorporated into the context of multiple use of lakes.
Management of water levels for flood control, water quality, and water safety purposes has become a priority for many lakes worldwide. However, the effects of water level management on the distribution and composition of aquatic vegetation has received little attention. Relevant studies have used either limited short-term or discrete long-term data and thus are either narrowly applicable or easily confounded by the effects of other environmental factors. We developed classification tree models using ground surveys combined with 52 remotely sensed images (15-30 m resolution) to map the distributions of two groups of aquatic vegetation in Taihu Lake, China from 1989-2010. Type 1 vegetation included emergent, floating, and floating-leaf plants, whereas Type 2 consisted of submerged vegetation. We sought to identify both inter- and intra-annual dynamics of water level and corresponding dynamics in the aquatic vegetation. Water levels in the ten-year period from 2000-2010 were 0.06-0.21 m lower from July to September (wet season) and 0.22-0.27 m higher from December to March (dry season) than in the 1989-1999 period. Average intra-annual variation (CV(a)) decreased from 10.21% in 1989-1999 to 5.41% in 2000-2010. The areas of both Type 1 and Type 2 vegetation increased substantially in 2000-2010 relative to 1989-1999. Neither annual average water level nor CV(a) influenced aquatic vegetation area, but water level from January to March had significant positive and negative correlations, respectively, with areas of Type 1 and Type 2 vegetation. Our findings revealed problems with the current management of water levels in Taihu Lake. To restore Taihu Lake to its original state of submerged vegetation dominance, water levels in the dry season should be lowered to better approximate natural conditions and reinstate the high variability (i.e., greater extremes) that was present historically.
Abstract— We experimentally tested the hypotheses that bluegills in vegetated habitats grow more rapidly than in nonvegetated habitats because (1) vegetated habitats contain a greater caloric density and (2) are less susceptible to energetic depletion. The 10-week experiment was conducted in enclosures containing factorial combinations of the presence or absence of Vallisneria americana and juvenile bluegills Lepomis macrochirus. After 6 weeks, Vallisneria-only treatments contained a mean of 1048 cal/m2 in the benthos, whereas treatments with both Vallisneria and bluegills contained 610 cal/m2. Hyalella azteca, a preferred prey of bluegill, were nearly depleted in nonvegetated enclosures, whereas Hyalella densities in enclosures with Vallisneria were much less effected by fish. Bluegill growth was significantly greater with Vallisneria than without but declining water temperatures after week 6 resulted in slower growth despite abundant prey. Ultimately, growth of bluegill resulted from an interaction between availability and ingestion of prey, and water temperature.
In order to reverse the damage to aquatic plant communities caused by multiple years of high water levels in Lake Okeechobee, Florida (U.S.), the Governing Board of the South Florida Water Management District (SFWMD) authorized a "managed recession" to substantially lower the surface elevation of the lake in spring 2000. The operation was intended to achieve lower water levels for at least 8 weeks during the summer growing season, and was predicted to result in a large-scale recovery of submerged vascular plants. We treated this operation as a whole ecosystem experiment, and assessed ecological responses using data from an existing network of water quality and submerged plant monitoring sites. As a result of large-scale discharges of water from the lake, coupled with losses to evaporation and to water supply deliveries to agriculture and other regional users, the lake surface elevation receded by approximately 1 m between April and June. Water depths in shoreline areas that historically supported submerged plant communities declined from near 1.5 m to below 0.5 m. Low water levels persisted for the entire summer. Despite shallow depths, the initial response (in June 2000) of submerged plants was very limited and water remained highly turbid (due at first to abiotic seston and later to phytoplankton blooms). Turbidity decreased in July and the biomass of plants increased. However, submerged plant biomass did not exceed levels observed during summer 1999 (when water depths were greater) until August. Furthermore, a vascular plant-dominated assemblage (Vallisnera, Potamogeton, and Hydrilla) that occurred in 1999 was replaced with a community of nearly 98% Chara spp. (a macro-alga) in 2000. Hence, the lake’s submerged plant community appeared to revert to an earlier successional stage despite what appeared to be better conditions for growth. To explain this unexpected response, we evaluated the impacts that Hurricane Irene may have had on the lake in the previous autumn. In mid-October 1999, this category 1 hurricane passed just to the south of the lake, with wind velocities over the lake surface reaching 90 km h-1 at their peak. Output from a three-dimensional hydrodynamic / sediment transport model indicates that during the storm, current velocities in surface waters of the lake increased from near 5 cm s
-1
to as high as 100 cm s
-1
. These strong velocities were associated with large-scale uplifting and horizontal transport of fine-grained sediments from the lake bottom. Water quality data collected after the storm confirmed that the hurricane resulted in lake-wide nutrient and suspended solids concentrations far in excess of those previously documented for a 10-year data set. These conditions persisted through the winter months and may have negatively impacted plants that remained in the lake at the end of the 1999 growing season. The results demonstrate that in shallow lakes, unpredictable external forces, such as hurricanes, can play a major role in ecosystem dynamics. In regions where these events are common (e.g., the tropics and subtropics), consideration should be given to how they might affect long-term lake management programs.
Macrophyte removal by lakefront property owners occurs on glacial lakes throughout the range of Largemouth Bass Micropterus salmoides, yet little information exists on how it affects recruitment of these fish populations. We hypothesized that with greater prey availability in macrophytes, age‐0 Largemouth Bass consumption and growth would increase. To test this hypothesis, we conducted an in‐lake experiment with age‐0 Largemouth Bass in twelve 9.29‐m2 littoral mesocosms in glacial Chancellor Lake, Michigan, during summer and fall 2014. We removed macrophytes from mesocosm substrates to produce low‐coverage, high‐coverage, and vegetation edge treatments and determined treatment effects on consumption and growth at differing age‐0 Largemouth Bass stocking densities (0.86–2.15 fish/m2). When macrophytes were present, age‐0 Largemouth Bass stomachs contained fewer zooplankton, contained terrestrial invertebrates more often, and contained aquatic macroinvertebrates less often than when macrophytes were absent. Age‐0 Largemouth Bass consumed more fish prey in macrophyte edge and high macrophyte coverage enclosures stocked at moderate densities, resulting in total stomach content biomass increases that increased growth and, potentially, recruitment. Age‐0 Largemouth Bass from enclosures with macrophyte edge and high macrophyte coverage grew 9.7 ± 2.2% (mean ± SD) and 8.3 ± 1.9% longer, respectively, than their counterparts in low macrophyte coverage enclosures; fish in macrophyte edge or high macrophyte coverage treatments at moderate stocking density gained 109.3 ± 45.1% and 110.7 ± 45.0% more weight, respectively, than those in low‐coverage enclosures. These results demonstrate a causal linkage between macrophytes and age‐0 Largemouth Bass consumption and growth in lakes. We recommend that fisheries managers seeking to increase consumption and growth rates in age‐0 Largemouth Bass consider protecting and restoring macrophytes in littoral habitats up to 60% coverage to potentially decrease mortality and improve recruitment.
Submerged macrophyte restoration was used to combat with lake eutrophication and improve water quality in a subtropical, shallow lake. A consistent investigation for three years about macrophyte, phytoplankton, zooplankton and water quality has been conducted to explore the effect of macrophyte restoration on improving aquatic ecosystem. The results showed that macrophyte biomass and coverage after the restoration were as high as 821 g/m² and 55%, respectively, and the dominant species were Ceratophyllum demersum, Myriophyllum verticillatum and Potamogeton crispus. Moreover, chemical oxygen demand (COD), total nitrogen (TN), total suspended solids (TSS) and nitrate nitrogen (NN) significantly decreased yearly, but the changes in total phosphorus (TP) and ammonia nitrogen (AN) were not significant during the investigation. Phytoplankton density and biomass significantly decreased, and the dominant species changed from blue-green algae to green algae. Zooplankton density and biomass, however, significantly increased yearly, and the small-sized dominant species (such as, Keratella cochlearis, Polyarthra trigla, etc.) were gradually replaced by the large-sized species (such as, Asplanchna priodonala, Alona guttata, etc.) during the restoration. In addition, macrophyte was negatively correlated with nutrients, but positively correlated with zooplankton. Phytoplankton was also positively related with TN. These correlations showed that restored macrophytes could decrease nutrition loading in water body, inhibit phytoplankton growth to decrease the risk of algae blooms, and support more large-sized zooplankton to strengthen the top-down control of phytoplankton. Therefore, our results showed that macrophyte restoration is a very potential and effective method to combat with the eutrophication and develop water quality in the eutrophic subtropical lakes, and deserves more attentions in future subtropical/tropical lake managements.
Understanding catchability—the fraction of a stock caught by a defined unit of effort—is crucial to using fisheries assessment data to index abundance. We conducted mark–recapture experiments to estimate catchability and evaluate standard boat electrofishing methods for assessing populations of Largemouth Bass Micropterus salmoides. We then used a resampling analysis to test for differences in bass CPUE (fish/h and fish/km) between two high-density reservoirs and one low-density reservoir and among surveys within each reservoir. We compared scenarios using surveys conducted only during (1) the standard time period (mid-April to mid-May) and (2) the entire assessment period (early April to mid-June). We considered the percentage of significant differences in CPUE between the high-density and low-density reservoirs to represent statistical power (i.e., the ability to detect a difference in CPUE when a difference actually exists) and the percentage of significant differences in CPUE between surveys in the same reservoir to represent the false-positive rate (i.e., the detection of a difference in CPUE when no difference in density exists). Catchability and CPUE were greatest and least variable during recapture events conducted during the standard period. The mean catchability of sub–stock length Largemouth Bass (150–200 mm) and memorable-length bass (≥510 mm) was significantly less than those for other length categories. Statistical power exceeded 80%, and the false-positive rate was generally less than 10% for sampling during the standard period at as few as six electrofishing sites. When including samples from outside the standard period, power was lower and the false-positive rate was as high as 60%. Power and false-positive rate were similar whether effort was measured in time or distance. Our results emphasize that standardized springtime boat electrofishing assessments validly index Largemouth Bass density and size structure.
Terrestrial and aquatic ecosystem degradation is widely recognized as a major global environmental and development problem. Although great efforts have been made to prevent aquatic ecosystem degradation, the degree, extent and impacts of this phenomenon remain controversial and unclear, such as its driving mechanisms. Here, we present results from a 17-year field investigation (1998–2014) of water quality and a 12-year remote sensing mapping (2003–2014) of the aquatic vegetation presence frequency (VPF) in Eastern Lake Taihu, a macrophyte-dominated bay of Lake Taihu in China. In the past 17 years, nutrient concentrations and water level (WL) have significantly increased, but the Secchi disk depth (SDD) has significantly decreased. These changes were associated with increased lake eutrophication and a degraded underwater light climate that further inhibited the growth of aquatic vegetation. In Eastern Lake Taihu, increased nutrients, chlorophyll a and WL, and a decreased SDD were all significantly correlated with a decreased VPF. NH4+-N concentration and SDD/WL were the most important controlling factors for VPF. Therefore, increased anthropogenic nutrient inputs and a degraded underwater light climate surely result in a decreased VPF. These results elucidate the driving mechanism of aquatic vegetation degradation and will facilitate Lake Taihu ecological restoration.
: Physical, chemical and biological influences of submersed vascular plants (dominated by Potamogeton perfoliatus and Ruppia maritima) on their surrounding environment are summarized for portions of upper Chesapeake Bay. Rates of accretion of organic matter in these ecosystems were high owing to the combined effects of vascular plant and associated algal production and the trapping of particulate organics of phytoplanktonic origin. Time-series observations of seston along transects traversing vegetated bottoms indicated significantly less turbid water over the plant beds, due both to increased deposition and to decreased resuspension of fine-grain sediments. Submersed plants provided a preferred habitat for many animal populations, and abundance of fishes (predominantly juveniles) was significantly greater in these plant beds than in adjacent unveqetated areas. Recent declines in several species of migrating waterfowl which feed directly on plant material were highly correlated with contemporaneous decreases in plant distribution. Rapid uptake of dissolved inorganic nitrogen (N) and phosphorus (P) was demonstrated for these communities, with subsequent incorporation into plant material via both growth and facultative increases in percent N and P composition. Upon senescence and death, submersed vascular plants decayed at moderate rates, with relatively slow releases of nutrients and low dissolved oxygen (O2) demand compared to algae (micro and macro) and to marsh grass. Thus, organic carbon (C) from these submersed plants is transferred to microbial food-chains, with minimal secondary effects of O2 depletion and nutrient enrichment. Part of the influence of these plant communities on the upper Bay is summarized in terms of three materials budgets for 1960, where these plants contributed 33% to the organic C budget, while acting as a seasonal sink for 210 and 7% of the total sediment and nitrogen inputs (respectively) to tne estuary.
Data from 56 Florida lakes were examined for relationships between abundance of aquatic macrophytes and young-of-the-year (< 160 mm TL), subadult (161 - 240 mm TL) and adult (>250 mm TL) largemouth bass (Micropterus salmoides). Study lakes ranged from 2 ha to 271 ha. Trophic status ranged from oligotrophic to hypereutrophic. The percentage of lake area covered (PAC) and the percentage of lake volume infested (PVI) with aquatic macrophytes among the lakes ranged from <1% to 100%. Young-of-the-year largemouth bass abundance ranged from 0 to 5857 fish/ha. Subadult largemouth bass abundance ranged from 0 to 216 fish/ha and adult largemouth bass abundance ranged from 1 fish/ha to 75 fish/ha. There were weakly significant, direct relationships among lakes between measures of macrophyte abundance and estimates of young-of-the-year and subadult abundance. There were weakly significant inverse relation ships among lakes between measures of macrophyte abundance and growth (mm/day) of age-1 and age-2 largemouth bass. There were no significant relationships among lakes between measures of macrophyte abundance and estimates of adult largemouth bass abundance or standing crop (kg/ha). Adult largemouth bass abundance and standing crop were positively correlated to lake trophic status. After accounting for lake trophic status, the abundance of young-of-the-year largemouth bass was directly related to PVI, and age-1 growth rate was inversely related to PVI but there were no significant relationships between macrophyte abundance and the abundance of subadult and adult largemouth bass. There are no strong predictable relationships between the abundance of aquatic macrophytes and the abundance of adult largemouth bass among Florida lakes < 300 ha.
Changes in electrofishing catch per hour (CPH) of age-0 largemouth bass Micropterus salmoides were examined in relation to aquatic macrophytes and seasonal water elevation at Lochloosa and Orange lakes, Florida, during the 1990s. At Lochloosa Lake, stepwise multiple regression revealed a significant positive relationship between the mean CPH of age-0 largemouth bass and the percentage of areal coverage by hydrilla Hydrilla verticallata. At Orange Lake, mean CPH was directly associated with the percentage of areal coverage by hydrilla and inversely related to summer water levels. Thus, the influence of vegetation on age-0 largemouth bass abundance was similar at both lakes, but the effects of water levels were not. Further investigations into the effects of fluctuations in water levels on age-0 largemouth bass in natural lakes are needed.
Publisher Summary This chapter discusses the factors controlling the zonation of freshwater plants. The groups of plants classified as macrophytes that comprise aquatic vegetation are presented along with their overall depth distribution. Specific examples of the zonation of vegetation in a range of lakes, to emphasize the distinction between environmental factors varying solely in the vertical plane and those that have both vertical and horizontal components are presented in the chapter. This distinction underlies the causal analysis of zonation. The chapter describes vertical sequences of growth form or individual dominants in relation to gradients in water table. The aquatic vegetation ranges along this gradient from woodland to submerged plant communities. The chapter discusses the habitat variables, vertical zonation, vertical and horizontal components of zonation, sediments and plant responses, plant adaptations, photomorphogenetic responses underwater, growth forms, plant strategies and interspecific competition, competition with microalgae, and the control of macrophyte zonation by substrate or by light.
Small areas that are barren of plants commonly occur amidst dense littoral zone vegetation of lakes. This study compared the benthic invertebrate communities of nonvegetated patches to those in the surrounding vegetated sediments in a Wisconsin lake. Mean benthic invertebrate densities in the sediments of Ceratophyllum demersum and Potamogeton nodosus beds were 13 and seven times, respectively, those of nearby nonvegetated areas within the littoral zone. The contrast in densities was especially striking for the gastropods Amnicola limosa and Gyraulus parvus, which were collected below Ceratophyllum at 162 times and 48 times, respectively, their densities in open areas. The conjecture that these density differences were due to a lack of a detrital food base in the nonvegetated areas was not supported by our investigation. The most likely explanations for such a contrast in invertebrate densities are: (1) disturbance effects in areas lacking plants, and (2) increased predator efficiencies in the open zones. Removal of macrophytes by herbicides, drawdown or mechanical means creates large expanses of open areas; our data indicate that macrophyte removal would result in a less diverse and much less abundant invertebrate fauna in littoral zone sediments. More than 90% of the invertebrates in 0-15 cm cores were found within the top 5 cm in nonvegetated areas as well as in sediments below a rooted macrophyte (Potomageton nodosus) and a nonrooted macrophyte (C. demersum). Despite the presence of its root system, which might oxidize the sediments, proportional abundances of invertebrates deeper in the sediments below Potamogeton were not greater than those below Ceratophyllum or in the open zones.
Lakes Weohyakapka and St. Johns Water Management Area, Florida, experienced severe impacts from multiple hurricanes in August and September 2004, resulting in the loss of all submersed aquatic vegetation, primarily hydrilla (Hydrilla verticillata). We assessed at both lakes changes in largemouth bass (Micropterus salmoides) population size distribution, recreational fishing effort and success, angler expenditures, and catches of trophy fish in relation to disparate levels of hydrilla coverage for prehurricane (1999–2004) and posthurricane (2005–2009) periods. Tests revealed significant differences at both lakes in the population size distribution between prehurricane (high percentage coverage of hydrilla) and posthurricane (no hydrilla) periods. At both lakes, the population size distribution comprised more juvenile (age-1) largemouth bass before the hurricanes, indicating that a decline in recruitment strength coincided with the absence of hydrilla posthurricanes. Declines in directed fishing effort, angler expenditures, and angler catches of trophy-sized fish also occurred following the absence of hydrilla posthurricanes. These findings demonstrate an important link between radical changes in hydrilla coverage with recruitment of juvenile largemouth bass and the strength of the largemouth bass fishery. The Florida Fish and Wildlife Conservation Commission recently adopted a new agency position on hydrilla management that allows flexibility regarding waterbodies with limited or absent native submersed vegetation, recognizing that hydrilla at a low to moderate coverage can be beneficial to fish and wildlife.
Survey data from 30 Texas reservoirs, collected between 1976 and 1978 as part of the Dingell-Johnson Reservoir Management Project, were analyzed to determine which factors affected largemouth bass (Micropterus salmoides) standing crops and their recruitment to harvestable size. A highly significant, positive relationship (P < 0.01) was found between percent submerged vegetation (up to 20%) and both the standing crop of largemouth bass and numbers being recruited to harvestable size. The relationship seemed to be linear within the range of values observed. Any reduction in submerged vegetation below 20% of the total lake coverage resulted in a decrease in recruitment and standing crop of largemouth bass. Conversely, to increase standing crop and recruitment of largemouth bass more than 10 in long in reservoirs having little or no cover, a program to increase submerged vegetation either through introductions or water-level manipulation should be implemented.
Assessment of the recruitment processes of juvenile largemouth bass Micropterus salmoides has involved a variety of gears, and little consideration has been given to the biases that gear size selection may introduce. To determine the influence of collecting methods on the interpretation of cohort characteristics, we compared length distributions of juvenile largemouth bass collected with a 9-m bag seine, a hand-held electrofisher, and a traditional boom-mounted electrofisher. The hand-held electrofisher was effective for sampling all lengths of fish up to 200 mm total length and sampled smaller fish than the boom-mounted unit. Seining was effective for sampling fish shorter than 60 mm but, relative to the hand-held electrofisher, it consistently underrepresented the contribution of fish longer than 70 mm to the cohort. The boom-mounted electrofisher was effective at sampling fish longer than 150 mm, but, compared with the hand-held electrofisher, it underestimated the contribution of smaller fish to population size structure. Gear selectivity may bias assessments of the importance of abundance and size to recruitment of juvenile largemouth bass.
In the remote sensing field, a frequently recurring question is: Which computational intelligence or data mining algorithms are most suitable for the retrieval of essential information given that most natural systems exhibit very high non-linearity. Among potential candidates might be empirical regression, neural network model, support vector machine, genetic algorithm/genetic programming, analytical equation, etc. This paper compares three types of data mining techniques, including multiple non-linear regression, artificial neural networks, and genetic programming, for estimating multi-temporal turbidity changes following hurricane events at Lake Okeechobee, Florida. This retrospective analysis aims to identify how the major hurricanes impacted the water quality management in 2003-2004. The Moderate Resolution Imaging Spectroradiometer (MODIS) Terra 8-day composite imageries were used to retrieve the spatial patterns of turbidity distributions for comparison against the visual patterns discernible in the in-situ observations. By evaluating four statistical parameters, the genetic programming model was finally selected as the most suitable data mining tool for classification in which the MODIS band 1 image and wind speed were recognized as the major determinants by the model. The multi-temporal turbidity maps generated before and after the major hurricane events in 2003-2004 showed that turbidity levels were substantially higher after hurricane episodes. The spatial patterns of turbidity confirm that sediment-laden water travels to the shore where it reduces the intensity of the light necessary to submerged plants for photosynthesis. This reduction results in substantial loss of biomass during the post-hurricane period.
Underwater macrophytes can influence fish diet and growth by restricting access inshore. The movements of largemouth bass (Microptemssalmoides) and biuegills (Lepomismacrochirus) were followed in a shallow Wisconsin impoundment nearly one-half covered with potamogetons and other macrophytes. Plant density averaged 130–200 g/m(dry weight) from May through August during the six-year study. Fish activity was observed from a still boat or by diving. Stomach samples of fishes were obtained on 55 dates by boom shocking. Submerged macrophytes at a density above 90 g/mfunctioned as a screen to selectively restrict fish movements. Bass and bluegills under age III were mostly confined to plant beds, where they grazed chironomid larvae and other aquatic insects on the plants and lake bottom. Bluegills shifted to zooplankton and finally plant tissue as summer progressed, while bass turned increasingly to fish as prey. Older bass had difficulty in penetrating the plant beds until openings appeared from plant decay or were created by a mechanical harvester. Slow growth of bass after age II was also reported in other Midwestern lakes with dense vegetation and is attributed to their difficulty in foraging on young fishes inshore. Selectively channelizing macrophyte beds with bottom screens or a mechanical harvester can be an effective management strategy to enhance fish growth in densely-planted lakes.
Hurricanes Frances and Jeanne (HF and HJ) passed over Lake Okeechobee, Fla., in September 2004. Strong winds produced a large surface seiche during both storms. The slope of the water surface reversed itself within 4 hrs as wind direction changed during HF, but shifted in only 2 hrs during HJ. The greatest water level difference was along the lake's north-south (N-S) axis during both storms (2.6 and 3.5 m, respectively). Differences between maximum wind set-up (storm surge) and set-down on the opposite shore indicated that the slope of the water surface was not symmetrical at the height of either storm. Using simple steady-state models, maximum wind set-up was forecast for opposing stations along the lake's N-S axis and compared to observed data and storm surge predictions by the SLOSH model. Steady-state model accuracy was not improved by using averaging periods >15 min in length or by lagging weather and water temperature data behind lake stage. Steady-state models calibrated to the data performed better than uncalibrated models. Prediction errors for maximum wind set-up during HF and HJ by SLOSH were comparable to errors in the steady-state models. While not a substitute for sophisticated hydrodynamic models like SLOSH and LOHM, properly calibrated steady-state models can provide lake managers with reasonable estimates of wind set-up. Future use of simple wind set-up models on Lake Okeechobee will require validation against data from other hurricanes.
Data from the literature suggest that predatory success declines as habitat complexity increases. To explain this phenomenon, we studied the predator-prey interaction between largemouth bass Micropterus salmoides and bluegills Lepomis macrochirus in four laboratory pools (2.4-3.0 m diameter, 0.7 m deep), each with a different density (0, 50, 250, 1,000 stems/m e) of artificial plant stems. Behavior was quantified for both predator and prey during largemouth bass feeding bouts lasting 60 minutes. Predation success (number of captures) by largemouth bass was similar at 0 and 50 stems/m 2, then declined to near zero at 250 and 1,000 stems/m 2. As stem density increased, predator activity declined due to a decrease in behaviors associated with visual contact with prey. Reduced predation success by largemouth bass in habitats of increased complexity apparently is related to increases in visual barriers provided by plant stems as well as to adaptive changes in bluegill behavior.
Hurricanes Frances and Jeanne passed over Lake Okeechobee, Florida, in September 2004 and Hurricane Wilma in October 2005. The storms created large waves, strong currents, high wind seiches and uplifted over 3 million metric tons (collectively) of sediments into the water column. Suspended solid concentrations increased five-fold and there were substantial changes in the plankton. Unlike previously documented effects of hurricanes in the open ocean and estuaries, where increased nitrogen inputs stimulate primary productivity, the hurricanes resulted in substantial reductions in biomass of bacteria, phytoplankton and phototrophic nanoflagellates, both in pelagic and near-shore habitats. Increases in macro-zooplankton biomass were observed in both habitats. There were sustained large increases in dissolved inorganic nitrogen and phosphorus in the water column after the hurricanes, coincident with large declines in mean irradiance in the mixed layer. Further, results from laboratory bioassays that exposed the phytoplankton to nutrient additions and a controlled light gradient indicate that the community shifted from being frequently nitrogen limited to most commonly light limited after the storms. The results confirm that the major driver of plankton food-web dynamics in this system is light availability, and that the primary mechanism of change caused by hurricanes is an accentuation of light limitation via greatly increased sediment re-suspension. There additionally was evidence of food-web-mediated effects where the loss of submerged vegetation and increased turbidity reduced the density and efficiency of visually feeding fishes, leading to a significant increase in biomass of macro-zooplankton.
This unique case study of Lake Okeechobee - a large, shallow and culturally eutrophic lake in south Florida - documents the effects of hurricanes on its water quality, sediment, phytoplankton and submersed aquatic vegetation (SAV). Three hurricanes (Frances and Jeanne in 2004 and Wilma in 2005) that swept directly over the lake led to a number of expected changes throughout the system: 1) high winds produced large seiches, strong waves and currents that redistributed bottom sediments and uprooted SAV and emergent macrophytes; 2) sediment disturbance resulted in increased suspended solids and nutrients in the water column, reduced Secchi transparency and affected SAV recovery, phytoplankton biomass and phytoplankton species dominance; and 3) heavy rainfall increased flows, nutrient loads and lake water levels. Changes in suspended solids, most nutrient concentrations, phytoplankton and SAV persisted for two years after the hurricanes. This persistence was attributed to unconsolidated surface sediment that increased in thickness because of the storms and was more easily resuspended during subsequent wind events. Drought conditions and low lake levels in the second year after the hurricanes led to some recovery of SAV, primarily in the form of the non-vascular musk grass (Chara spp.). The absence of high-intensity hurricanes in the near future should aid in SAV recovery and return the nearshore region to a macrophyte-dominated clear-water state. Our results demonstrate the importance of sediment disturbance and water levels in shallow lakes that are vulnerable to extreme weather events.
Human modifications to ecosystems can excerbate effects of natural disturbances, but interactions between disturbance and altered hydrology are rarely assessed. Lake Okeechobee, Florida, was impacted by four large hurricanes in 2004–2005 that caused large lake-wide reductions in the coverage and biomass of aquatic macrophytes. We quantified dramatic changes in the aquatic plant and littoral fish assemblage after hurricanes, including decreased fish richness, diversity, and biomass, and large reductions in the species that support important fisheries. Human-induced hydrologic changes prevented the lake from expanding to the historically connected floodplain during and following hurricanes, which likely exacerbated impacts to aquatic plant and fish communities. Altered hydrology has complicated management scenarios for returning the lake to pre-hurricane conditions because policy options that will initiate recovery at Lake Okeechobee (i.e., lower water levels) will exert negative impacts on east and west coast Florida estuaries. Anthropogenic modifications raise difficult trade-offs for ecosystem restoration.
Juvenile largemouth bass Micropterus salmoides were collected by electrofishing during October through March 1992–1994 from coves (≤25 ha) covered with aquatic macrophytes over 1–65% of their area. Mean total length of juvenile largemouth bass was highest in coves with the least vegetated cover, but increase in mean length between October and March was highest in coves having near 20% vegetation coverage. Catch per unit effort decreased between October and March; decreases were least at vegetation coverages near 10–20%, highest at coverages of 5% or less, and intermediate at coverages of 30–65%. By March, these disparate decreases contributed to the formation of a dome-like relationship between vegetation coverage and catch per unit effort. Consumption of fish foods was highest when vegetation coverage was low, but decreased asymptotically as coverage increased; consumption of invertebrate foods increased at low coverage, peaked near 20–30% coverage, and decreased at higher coverage. We suggest that greater length increases and greater abundance at 10–25% vegetation coverage were stimulated by a favorable blend of food availability and cover. Our results support reports that maximum recruitment of largemouth bass occurs at intermediate levels of vegetation coverage, and we further suggest that such increased production is reinforced during winter, when survival. invertebrate consumption, and length increases are highest at intermediate levels of vegetation coverage.
Lake Tåkern and Lake Krankesjön, two moderately eutrophic, shallow lakes in southern Sweden, have during the past few decades shifted several times between a clear‐water state with abundant submerged vegetation and a turbid state with high phytoplankton densities.
Between 1985 and 1991, Lake Takern was in a clear state, whereas Lake Krankesjon shifted from a turbid to a clear state. During this shift, the area covered by submerged macrophytes expanded, followed by an increase in water transparency, plant‐associated macroinvertebrates, and piscivorous fish. Nutrient concentrations, phytoplankton biomass and abundance of planktonic cladocerans decreased.
In both lakes, water level fluctuations were the most common factor causing shifts, affecting submerged macrophytes either through changes in light availability or through catastrophic events such as dry‐out or mechanical damage by ice movement.
Our data give further support for the existence of two alternative stable states in shallow lakes maintained by self‐stabilizing feedback mechanisms.