A Model to Estimate Potential Submersed Aquatic Vegetation Habitat Based on Studies in Lake Pontchartrain, Louisiana

The depth distribution of submersed aquatic vegetation (SAV) was studied in Lake Pontchartrain, Louisiana, to develop a model to predict changes in SAV abundance from changes in environmental quality. We conducted annual line‐intercept surveys from 1997 through 2001 and monitored monthly photosynthetically active radiation at four sites with different shoreface slopes. The following relationships between SAV distribution and environmental factors were used as model parameters: (1) water clarity controls SAV colonization depth; (2) fluctuation in annual mean water level and wave mixing determines SAV minimum colonization depth; and (3) site differences in SAV areal coverage under the comparable water quality conditions are due to shoreface slope differences. These parameters expressed as mathematical components of the model are as follows: mean water clarity determines SAV colonization depth (Z_max= 2.3/K_d); mean water level and wave mixing controls SAV minimum depth (Z_min= 0.3 m); and shoreface slope angle (θ) determines the distance from Z_min to Z_max. The equation developed for the potential SAV habitat (PSAV) model is PSAV = (2.3 ? 0.3 ×K_d)/(sinθ×K_d). The model was validated by comparing empirical values from the dataset to values predicted by the model. Although the model was developed to predict the PSAV in Lake Pontchartrain, it can be applied to other coastal habitats if local SAV light requirements are substituted for Lake Pontchartrain values. This model is a useful tool in selecting potential restoration sites and in predicting the extent of SAV habitat gain after restoration.     

Aquatic vegetation and largemouth bass population responses to water-level variations in Lake Okeechobee,Florida (USA)

A five-year study examined the responses of submerged aquatic vegetation (SAV), emergent vegetation, and largemouth bass (Micropterus salmoides) to variations in water level in a large lake in Florida, USA. SAV was assessed using a combined transect survey/spatial mapping approach, emergent vegetation was quantified with aerial photography and GIS, and bass were surveyed by electro-shocking. During the period leading up to this study (1995–1999), water levels were high in the lake, and the SAV was reduced in spatial extent and biomass, compared to its condition in the early 1990s. Spatial extent of emergent vegetation also was low, and largemouth bass surveys indicated low densities and failure to recruit young fish into the population. This was attributed to the lack of critical vegetative habitat. In spring 2000, the lake was lowered by discharging water from major outlets, and this was followed by a regional drought. Water levels dropped by 1m, and there was widespread development of Chara lawns in shoreline areas, with coincident increases in water clarity. There was some germination of vascular SAV, but Chara was the extreme dominant, such that structural complexity remained low. There was no substantive improvement in bass recruitment. During 2001, water levels declined further, and emergent plants germinated in exposed areas of the lake bottom. SAV was restricted to sites farther offshore, and continued to be dominated by Chara. There again was no bass response. In 2002, conditions changed when water levels increased to a moderate depth, flooding shoreline habitat to 0.5m. Vascular SAV increased in biomass and spatial extent, such that the community developed a high structural complexity. At the same time, emergent aquatic plants developed dense stands along the western shoreline. Largemouth bass had a strong recruitment of young fish for the first time in 5years. Recruitment continued to be successful in 2003, when spatial extent of SAV was somewhat reduced by higher water but total biomass and diversity remained high. These results demonstrate an important effect of inter-annual variation in water depth on the population dynamics of aquatic plants and fish in a subtropical lake.     

A conceptual ecological model to aid restoration of Cootes Paradise Marsh,a degraded coastal wetland of Lake Ontario,Canada

An ecological model is derived from recent studies, based on 60 years of empirical observations and experimental data, that conceptualizes how Cootes Paradise Marsh was transformed from a lush emergent marsh with considerable ecological diversity in all trophic levels, to one that is currently turbid, devoid of vegetation, and dominated by a few exotic plant and fish species. This conceptual model contains 17 key components that interact and contribute to the overall unhealthy state of the marsh. The most influential component is high water level which caused the initial loss of emergent vegetation in the 1940s and 1950s. In the absence of plants to attenuate sediment and assimilate nutrients, the marsh became turbid and windswept, and this led to the disappearance of submergent vegetation over the next two decades. Currently, high water turbidity is being maintained by wind re-suspension, high sediment loading from the watershed during the summer, high algal biomass resulting from excessive nutrient loads from sewage effluent and surface runoff, and the feeding and spawning activities of a very large population of common carp ( Cyprinus carpio). Due to vegetation loss, the substrate has become mostly loose sediment that is no longer suitable for the diverse assemblage of aquatic insect larvae that lived on the plants and detrital material in the 1940s. Benthic grazers have been kept in low abundances due to predation by benthivorous carp; consequently, epiphytic algae have proliferated and further contribute to light limitation of macrophytes. High nutrient loadings contribute to high diurnal fluxes in dissolved oxygen levels that tend to select against less tolerant organisms such as insect larvae (other than chironomids) and piscivores (northern pike and largemouth bass). Without piscivores in the marsh, the planktivores have become dominant and have virtually eliminated all of the large herbivorous zooplankton (e.g., Daphnia), except for a few pockets in the marsh inlets close to residual macrophyte beds. Because of the dominance of small-bodied inefficient grazers (rotifers and small cladocerans), algal biomass is high, and the community has a large proportion of heterotrophic forms that tolerate low light environments. This ecological model suggests that the current turbid un-vegetated state of Cootes Paradise may be very stable. It will persist as long as water levels remain unfavorable for natural re-colonization by the emergent flora, and/or water turbidities remain sufficiently high to suppress the growth of submergent vegetation. Using this conceptual model, I developed a model of how Cootes Paradise Marsh may have functioned as a healthy marsh prior to the 1940s, and use these models as a basis to explore a number of restoration and management options and discuss their implications on the aquatic foodweb.     

Long-term GIS-based records of habitat changes in a Lake Erie coastal marsh

Great Lakes wetlands have lost much of their historical extent, structure and function. Their transformation was influenced by a number of factors acting over a period of decades including modifications in the basin's hydrology (watershed drainage, dikes, lake levels), biology (exotic species), geology (sediment transport and composition), and chemistry (water quality). The relative importance of each of these catalysts likely varied from region to region and depended on pre-settlement conditions and natural variability in the marshes, both generally unknown. We applied Geographic Information Systems (GIS) technology to a 120-year record (1872–1991) of images of a 2000-ha marsh system along the southwestern shore of Lake Erie, Ohio, USA. Long-term variability in aggregate characteristics of wetland vegetation was linked with environmental changes and human impact in three regions representing (1) a naturally existing open marsh with the lowest relative topograhical elevation, (2) an open marsh with a protected exposure to the lake and higher elevation, and (3) a diked marsh with manipulated water levels. The deep, open marsh lost half of its emergents, and a third of its patchiness and edge habitat in the early 1900s, when severe watershed degradations accompanied relatively low Lake Erie water levels. Nearly all remaining emergents were eliminated between 1940 and 1991 in this open marsh, following progressively higher lake levels. In the protected open marsh, the extent of emergents fluctuated with lake levels until 1977, and declined severely since then during sustained high lake levels. Habitat parameters varied little until recent decades, but declined markedly thereafter. The diked marsh maintained pre-1900 conditions for emergent plants, patchiness, and habitat edge; variability was linked to breached dikes and the presence/absence of marsh management. No landward re-establishment of the entire marsh complex since 1873 was evident on the 10 images studied. We propose the interaction of three forcing functions as the principal mechanism controlling the historical and current distribution of aquatic plants in southwestern Lake Erie marshes. These factors, collectively referred to as the Landward Advance Paradigm (LAP), include (1) the restricted ability of the marshes to advance landward, (2) sustained above-average lake levels, and (3) the presence of carp ( Cyprinus carpio) in wetlands with silt and clay sediments. Management focused on enhancing the role of Lake Erie wetlands should consider these landscape-level alterations and attempt to maximize wetland functions within the context of the LAP.     

Submerged aquatic vegetation correlations with depth and light attenuating materials in a shallow subtropical lake

A 3-year study was done to quantify the biomass of submerged aquatic vegetation (SAV) and its relationship with environmental attributes in Lake Okeechobee, the largest lake in the southeastern United States. Plants were sampled on 21 occasions at sites located along 15 fixed transects around the shoreline, giving rise to 721 observations of SAV species (Chara spp., Vallisneria americana, Hydrilla verticillata, Potamogeton illinoinensis) dry weight biomass. Environmental sampling focused on factors that attenuate light, including phytoplankton chlorophyll a (chl a), total suspended solids (TSS), non-volatile suspended solids (NVSS) and color. Depth and Secchi transparency also were measured. Based on regression analysis, NVSS was considerably more important in attenuating light than chl a or color. Total biomass of SAV varied from 0 to 271 g dw m^–2, with a mean of 4.7 g dw m^–2, and strong dominance by Chara. The SAV biomass was lower than average for Florida lakes, and may reflect the influence of suspended solids on underwater irradiance, as well as high water level in the late 1990s. Dense SAV was found only where depth was < 2 m and TSS < 20–30 mg l^–1. At locations where high biomass of SAV occurred, the plants may have influenced water quality, because concentrations of TSS, NVSS, and chl a were 2–3 fold lower than at sites with no plants. The potential effects of SAV also were apparent at a regional scale. The shoreline region of the lake displayed a pattern of rising and falling chl a and NVSS with water depth. This occurred both at sites with and without plants, suggesting that it may be driven by physical processes, such as water circulation patterns, which are influenced by depth. However, the pattern was dampened at sites with SAV, indicating a potential to influence these attributes of water quality.     

光衰减及其相关环境因子对沉水植物生长影响研究进展

光衰减对沉水植物的生长具有至关重要的影响。系统归纳总结了光衰减及其相关环境因子对沉水植物生长的影响,指出:光因子是沉水植物生长的第一环境要素,水体中的有色可溶性有机质、浮游植物叶片细胞中的叶绿素和悬浮颗粒物以及水体本身,对光穿透水体时光强的衰减有着直接的影响,是影响沉水植物最重要的光衰减水质参数。其他环境因子如营养盐、沉积物和流水动力学等因素,则会直接或间接影响光衰减水质参数,进而影响水体透明度和浑浊度,影响沉水植物的光合作用,是影响沉水植物光衰减的间接环境因子。提出了研究中重点关注的几个问题。     

Aquatic insect emergence in two Great Lakes marshes

This study determined total number, biomass, taxa, and seasonal occurrence of adult aquatic insects emerging from four vegetation zones in one diked and one undiked freshwater coastal marsh on hypereutrophic lower Green Bay, Lake Michigan, USA during the summer of 1984. Floating box traps were placed in open water, sparse emergent, dense emergent, and wet meadow vegetation zones in each marsh. Insects were collected during 20 24-hour periods, each four days apart, from June 11 to August 26. Two-way ANOVA was used to test differences in number and biomass of insects between marshes and among vegetation zones. Polynomial regression was used to evaluate seasonal emergence patterns. More insects, insect biomass, and insect taxa were found in the diked marsh, especially during the first half of the sampling period. Damselflies were much more abundant in the diked marsh. Most insects and insect biomass were found in the sparse emergent vegetation zone of both marshes. The emerging insect community in the diked marsh appears enhanced by its separation from the hypereutrophic and turbid waters of lower Green Bay.     

North African wetland lakes: characterization of nine sites included in the CASSARINA Project

Exploitation of land and water resources has increased rapidly in North Africa during the 20th century, paralleling regional population growth. As part of the CASSARINA Project (see Flower, 2001), the environmental status of nine wetland lakes in Morocco, Tunisia, and Egypt was evaluated. All are conservationally important habitats and several are Ramsar Sites (internationally recognized bird reserves) and several support significant fisheries. All are shallow (<2 m in depth) but vary greatly in area.Where available, documentary information on relevant 20th century changes is given. Survey transects for aquatic vegetation were established and used to provide baseline ecological information on the aquatic plant communities during 1997–1999. Unusually, one site (Tunisian Megene Chitane) supported acidophilous vegetation (some taxa being nationally rare). Aquatic macrophytes declined catastrophically at two sites during the 1990s. Merja Bokka was drained in 1998 and, at Garaet El Ichkeul, fringing Phragmites and Scirpus spp. were lost, mainly as a result of salinity changes. Elsewhere, fringing macrophytes remain (extensively so in the Nile Delta lakes) common, despite major land reclamation and water quality problems, or are degraded by grazing (Merja Zerga). Marginal vegetation during 1997/98 changed markedly at Megene Chitane due to water level lowering.Documentary records indicated that throughout the 20th century, reclamation and hydrologic modifications, mainly for agricultural purposes, affected all nine sites. The loss of lake area by reclamation is substantial for the Nile Delta lakes (Edku, Burullus and Manzala). For the western sites, some data indicate increasing salinity in the most recent decade but the Delta lakes have become generally fresher during the 20th century, as supply of Nile water for irrigation increased.Despite intense human disturbance, many of the remaining CASSARINA sites still support regions of high aquatic diversity. Spatial scale monitoring of the larger sites for seasonal and inter-annual changes in open water area and in aquatic plant abundances is a key requirement for integrated environmental change assessment in the 21st century.     

1998年特大洪水后鄱阳湖自然保护区主要湖泊水生植被的恢复

鄱阳湖自然保护区的湖泊是相对独立于鄱阳湖主体湖的一个区域,是国际重要湿地。1998年的特大洪水导致湖泊中水生植物的地上部分大量毁灭。通过1999年和2001年的植被调查,并与历史资料比较,探讨了特大洪水干扰后的植被恢复动态。结果表明,1999年湖泊水生植物的种类和生物量均低于干扰前的水平;2001年物种种类已经恢复,苦草(Vallisneria spp．)和黑藻(Hydrialla verticillata)的生物量已超过干扰前的水平,但其它物种的生物量仍较低,尚处于恢复的初始阶段。据此推断,物种问恢复速度的差异主要与物种的无性繁殖方式有关。鄱阳湖自然保护区湖泊的植被恢复不同于温带和其它亚热带的湖泊,不经历轮藻(Chara spp．)作为先锋优势种的阶段,苦草和黑藻可以作为先锋种首先在湖泊中恢复。这可能与鄱阳湖作为通江湖泊其水位频繁波动、轮藻不易定居有关。研究显示,洪水导致的水生植物生物量下降和物种数目减少只是短期现象,湖泊水生植物能在几年内恢复到干扰前的水平。     

Effects of a Prorocentrum minimum bloom on light availability for and potential impacts on submersed aquatic vegetation in upper Chesapeake Bay

Extraordinary spring blooms of the dinoflagellate Prorocentrum minimum have been a recurring feature of upper Chesapeake Bay for many years. Though not thought to be toxic in Chesapeake Bay, these blooms produce extraordinarily high concentrations of chlorophyll, thereby increasing light attenuation. A particularly large event occurred in the spring of 2000. Here, we assess the impact of the spring 2000 P. minimum bloom on habitat quality for submerged aquatic vegetation (SAV) in the mesohaline region of Chesapeake Bay and its tributaries. We determined the light absorption and scattering spectrum of P. minimum on a per cell basis by analyzing inherent optical properties of natural samples from the Rhode River, Maryland, which were overwhelmingly dominated by P. minimum. Using these per cell properties, we constructed a model of light penetration incorporating observed cell counts of P. minimum to predict the impact of the bloom on other tributaries and main stem locations that experienced the bloom. Model estimates of diffuse attenuation coefficients agreed well with the limited measurements that were available. Impacts of the mahogany tide on diffuse attenuation coefficient ranged from negligible (10–30% increase above the seasonal median in the Patapsco and Magothy rivers), to a greater than six-fold increase (Potomac River). Attenuation coefficients in tributaries to the north and south of the bloom region either decreased or were unchanged relative to seasonal medians. Segments with SAV losses in 2000 were mostly the same as those that experienced the P. minimum bloom. Segments north and south of the bloom area mostly had SAV increases in 2000. Though all of the segments that experienced a decline in SAV area after the spring 2000 bloom showed an increase in 2002, the 2000 setback interrupted what otherwise has been a slow recovery in mid-Bay SAV, demonstrating the adverse impact of P. minimum blooms on SAV populations in Chesapeake Bay.     

Rapid responses of vegetation to hydrological changes in Taylor Slough, Everglades National Park, Florida, USA

We analyzed the dynamics of freshwater marsh vegetation of Taylor Slough in eastern Everglades National Park for the 1979 to 2003 period, focusing on cover of individual plant species and on cover and composition of marsh communities in areas potentially influenced by a canal pump station (“S332”) and its successor station (“S332D”). Vegetation change analysis incorporated the hydrologic record at these sites for three intervals: pre-S332 (1961–1980), S332 (1980–1999), post-S332 (1999–2002). During S332 and post-S332 intervals, water level in Taylor Slough was affected by operations of S332 and S332D. To relate vegetation change to plot-level hydrological conditions in Taylor Slough, we developed a weighted averaging regression and calibration model (WA) using data from the marl prairies of Everglades National Park and Big Cypress National Preserve. We examined vegetation pattern along five transects. Transects 1–3 were established in 1979 south of the water delivery structures, and were influenced by their operations. Transects 4 and 5 were established in 1997, the latter west of these structures and possibly under their influence. Transect 4 was established in the northern drainage basin of Taylor Slough, beyond the likely zones of influence of S332 and S332D. The composition of all three southern transects changed similarly after 1979. Where muhly grass (Muhlenbergia capillaris var. filipes) was once dominant, sawgrass (Cladium jamaicense), replaced it, while where sawgrass initially predominated, hydric species such as spikerush (Eleocharis cellulosa Torr.) overtook it. Most of the changes in species dominance in Transects 1–3 occurred after 1992, were mostly in place by 1995–1996, and continued through 1999, indicating how rapidly vegetation in seasonal Everglades marshes can respond to hydrological modifications. During the post-S332 period, these long-term trends began reversing. In the two northern transects, total cover and dominance of both muhly grass and sawgrass increased from 1997 to 2003. Thus, during the 1990’s, vegetation composition south of S332 became more like that of long hydroperiod marshes, but afterward it partially returned to its 1979 condition, i.e., a community characteristic of less prolonged flooding. In contrast, the vegetation change along the two northern transects since 1997 showed little relationship to hydrologic status.     

Post‐storm sediment burial and herbivory of Vallisneria americana in the Hudson River estuary: mechanisms of loss and implications for restoration

This article investigates the mechanics of loss of Hudson River Vallisneria americana after the high volume storms at the end of the 2011 growing season, when two severe weather events—Tropical Storm Irene and the remnants of Tropical Storm Lee—struck the Hudson River watershed. In 2012, the distribution of the most common species of submerged aquatic vegetation (SAV), Vallisneria americana (wild celery, water celery, or tape grass), in the Hudson River estuary declined by more than 90%, with no appreciable recovery in 2013 and 2014. Because of its important habitat value for aquatic life and for increasing dissolved oxygen, managers and scientists have begun discussing the reasons for the loss, as well as how to assist its recovery through assisted restoration efforts in the estuary. Supported by in situ and in vitro experiments, the article posits the hypothesis that sediment, washed into the river by the storm, buried overwintering tubers of the plant, thus reducing sprouting success. Sprouting was as low as 50% with sediment depth between 2 and 5 cm; sprouting did not occur with sediment depth greater than 10 cm. Field experiments found no support for the hypothesis that herbivory inhibited regrowth of the plant after the storm events. These results suggest that future assisted restoration of Vallisneria americana and SAV in general may require attention to system‐specific factors.     

Plant community,primary productivity,and environmental conditions following wetland re-establishment in the Sacramento-San Joaquin Delta,California

Wetland restoration can mitigate aerobic decomposition of subsided organic soils, as well as re-establish conditions favorable for carbon storage. Rates of carbon storage result from the balance of inputs and losses, both of which are affected by wetland hydrology. We followed the effect of water depth (25 and 55 cm) on the plant community, primary production, and changes in two re-established wetlands in the Sacramento San-Joaquin River Delta, California for 9 years after flooding to determine how relatively small differences in water depth affect carbon storage rates over time. To estimate annual carbon inputs, plant species cover, standing above- and below-ground plant biomass, and annual biomass turnover rates were measured, and allometric biomass models for Schoenoplectus (Scirpus) acutus and Typha spp., the emergent marsh dominants, were developed. As the wetlands developed, environmental factors, including water temperature, depth, and pH were measured. Emergent marsh vegetation colonized the shallow wetland more rapidly than the deeper wetland. This is important to potential carbon storage because emergent marsh vegetation is more productive, and less labile, than submerged and floating vegetation. Primary production of emergent marsh vegetation ranged from 1.3 to 3.2 kg of carbon per square meter annually; and, mid-season standing live biomass represented about half of the annual primary production. Changes in species composition occurred in both submerged and emergent plant communities as the wetlands matured. Water depth, temperature, and pH were lower in areas with emergent marsh vegetation compared to submerged vegetation, all of which, in turn, can affect carbon cycling and storage rates.     

干旱对湖北省长湖水生植物多样性的影响

将断面法和地理信息系统（GIS）与全球定位系统（GPS）技术相结合,通过比较湖北省长湖2000年（干旱年）与1999年（非干旱年）的水生植物多样性,探讨了干旱对淡水湖泊水生植物多样性的影响,主要结论如下：（1）干旱对长湖淡水湖泊水生植物物种多样性无影响,但可增加优势种数目（从12个至14个）。（2）干旱使长湖水生植物群丛数目从14个增加至18个,且能显著提高各群丛的Simpson与Shannon-Wiener物种多样性指数。（3）干旱显著提高了长湖水生植被覆盖率与各群丛的盖度,并使全湖水生植被平均单位面积生物量与各群从单位面积生物量显著升高。（4）在干旱年（2000年）,长湖挺水植被消失,但浮水植被面积与沉水植被面积显著扩大：分别从1999年的3.71％与41.32%上升至2000年的12.63%与53.84%（占湖泊总面积的百分比）。（5）干旱条件下淡水湖泊水生植物的生长发育明显加快：2000年7月菹草石芽萌发率、菹草幼株长度和野菱黄叶率显著高于1999年同期值。     

Setting reclamation targets and evaluating progress: Submersed aquatic vegetation in natural and post-oil sands mining wetlands in Alberta, Canada

Oil sands mining disturbs thousands of hectares of boreal landscape, about 65% of which is wetland. Its reclamation will constitute the largest wetland reclamation project in Canadian history. We developed a unified analytical framework that we used to set reclamation targets and evaluate reclamation progress using submersed aquatic vegetation (SAV). We sampled SAV in 38 minimally disturbed wetlands to establish a reference condition and compared this to SAV in 25 reclamation wetlands. We observed 26 taxa: all were native and five are regionally rare. Using a combination of ordination, clustering, and indicator species analysis, we identified seven SAV assemblages, distinguishable based on 10 indicator species. The assemblages found in wetlands contaminated by tailings had significantly fewer taxa. Using joint plots, we demonstrate that they differ in terms of environmental variables reflecting depth, slope, salinity, transparency, water and sediment nutrient levels, and alkalinity. Collectively, 74% of reference wetlands had SAV belonging to either marsh or fen-marsh assemblages that we consider suitable targets for reclamation. Having multiple reclamation targets avoids creating a homogenous post-mining landscape with low gamma diversity. Using complementary multivariate and categorical tests, we found that reclamation wetlands failed to support either of these dominant reference assemblages. Instead, every reclamation wetland supported one of five atypical SAV assemblages, each of which was uncommon (<10%) among reference wetlands. The analytical framework we developed can be applied to other ecosystems or taxa to develop targets for reclamation, to evaluate reclamation progress, and to identify environmental characteristics associated with reclamation targets.     

Effects of structural marsh management on fishery species and other nekton before and during a spring drawdown

We sampled experimental research areas in the Barataria Basin of Louisiana, USA to examine the effects of structural marsh management on habitat use by small nekton (>100 mm Total Length or Carapace Width). The research areas consisted of two control (unmanaged) marshes and two impounded (managed) marshes; managed areas were surrounded by levees with water-control structures constructed by the U.S. Department of Interior, National Biological Survey. We sampled nekton with 1-m² enclosure samplers in 1995 just as a drawdown was initiated (March) and after two months of drawdown (May); a drawdown is an active management technique in which water is allowed to flow out of, but not back into, the impoundment. Samples were collected randomly from all available habitat types (shallow open water, submerged aquatic vegetation (SAV), and intertidal marsh) in the managed and unmanaged areas. In March, the densities of resident taxa (e.g., Lucania parvac rainwater killifish and Palaemonetes paludosus riverine grass shrimp), which complete their life cycles within the estuary, were significantly greater in the managed areas compared to the unmanaged areas. The densities of most resident species were either similar in managed and control areas, or significantly greater in control areas during the drawdown (May). In contrast to residents species, the transient fishery species (e.g., Callinectes sapidus blue crab and Farfantepenaeus aztecus brown shrimp) reproduce outside of the marsh system and recruit to these areas as young. The densities of these transient species were significantly higher in unmanaged areas compared to managed areas during both sampling periods. We estimated standing crop (number or biomass of nekton per hectare of marsh area) by combining habitat densities with the area of different habitat types. The standing crops of transient species also were substantially greater in unmanaged than managed areas. We conclude that the restricted water exchange in marshes under structural marsh management diminishes recruitment and standing stocks of species that must migrate from coastal spawning sites to marsh nurseries. Even when water-control structures were open, the densities of these transient species were low inside managed areas. In contrast to the negative effect of management on transient species, the resident fish and crustacean populations seemed to flourish in the managed areas when a drawdown was not in effect. Following two months of a drawdown, however, the populations of residents appeared similar inside and outside managed areas. Increases in submerged aquatic vegetation (SAV) within ponds occurred outside the managed areas during the study period, but not inside managed areas. Because many resident species were closely associated with the SAV, the effect of management on SAV may have been responsible for the distribution patterns of resident species.     

Changes in Diet and Food Consumption of Largemouth Bass Following Large-scale Hydrilla Reduction in Lake Seminole, Georgia

Largemouth bass Micropterus salmoides Lacepède growth (in length) increased an average of 14% and bioenergetics modeling predicted a 38% increase in total annual food consumption following a large-scale reduction of hydrilla Hydrilla verticillata L.f. Royle in Spring Creek, a 2,343-ha embayment of Lake Seminole, Georgia. Coverage of submersed aquatic vegetation (SAV) declined from 76% to 22% in 1 year due to a drip-delivery fluridone treatment. In contrast, largemouth bass growth only increased an average of 4% and bioenegetics modeling predicted a 13% increase in total food consumption over the same time period in the Chattahoochee River embyament, where SAV coverage naturally declined from 26% to 15%. Diets were collected from a total of 4,409 largemouth bass over a 2.5-year period in the two embayments; the primary diet item (by weight) for largemouth bass in both embayments was sunfish (mostly Lepomis spp.). Diets before and after SAV reduction were generally similar for fish greater than stock-size (≥203 mm) in the Spring Creek arm; however, fewer invertebrates were consumed after SAV reduction. Low diet similarity was observed in smaller fish, caused by a decline in consumption of grass shrimp and sunfishes and an increase in use of damselflies, shiners Notropis spp., and topminnows Fundulus spp. after SAV reduction. Diets were similar between the same time periods for all sizes of fish in the Chattahoochee River arm. These results agreed with many laboratory results describing the effects of aquatic plant density on largemouth bass food consumption and growth, and demonstrated that increased predation efficiency resulting from decreased plant abundance was likely a stronger factor determining growth rates than any potential diet shift that may occur as a result in vegetation decline.     

Spatial and temporal changes in species composition of submersed aquatic vegetation reveal effects of river restoration

We examined temporal changes in spatial patterns of submersed aquatic vegetation (SAV) in response to the restoration of geomorphic habitat in Navigation Pool 8 of the Upper Mississippi River from 1998 to 2016. The frequency of occurrence and species composition of SAV at sampling sites were spatially interpolated for each year to create annual maps. Linear models were fitted to temporal changes in SAV within each map pixel. The frequency of occurrence of SAV (across all species) increased over time in much of the impounded region of the pool, including areas near restored islands. However, impounded areas maintained a relatively consistent species composition over time, with species known to be tolerant of higher flow velocities (>0.10 m/second) and wind fetch distances (>1,000 m) (e.g. Vallisneria americana) being most abundant. In contrast, areas protected by newly constructed islands transitioned from V. americana to species found in other protected backwater habitats and known to be intolerant of high flow velocities and wind fetch distances (e.g. Ceratophyllum demersum). The results suggest that previously reported improvements in water clarity may have improved growing conditions for all SAV species, especially in the lower impounded portion of the pool, while island restoration created more backwater‐like habitats and facilitated changes in species composition. Assessing changes in SAV occurrence alone offers only a partial view of local‐scale river restoration (e.g. island building), while analyses of species composition are likely to be more indicative of the types of changes (i.e. reduced flow velocity and wind fetch) associated with restoring geomorphic habitat.     

Floating mats: their occurrence and influence on shoreline distribution of emergent vegetation

1. A study was conducted on the northern shore of Lake Victoria (Uganda) to determine the factors controlling the occurrence of floating root mats and the influence of the floating mats on the distribution of emergent vegetation. 2. Environmental conditions within 78 bays in the study area were characterised using bay size, wave exposure, water depth, littoral slope, sediment characteristics and water level fluctuations. Emergent plants that form floating root mats occur along the shores of these bays. The way in which commonly occurring shoreline vegetation was distributed across a wave‐exposure gradient was compared with their distribution across a water level fluctuation gradient. 3. Results suggested that wind–wave action and water level fluctuations are important factors determining the occurrence of floating mats. Mat‐forming plants occur in the most sheltered locations along the shore and in waterbodies with modest water level fluctuations. 4. The ability to form mats facilitated the lakeward expansion of emergent plants. Plants forming floating root mats had a larger depth range than non‐mat forming plants. 5. The initiation mechanisms for the floating mats of emergent vegetation in Lake Victoria appear to be: (i) invasion of mats of free‐floating plants by emergent vegetation; and (ii) detachment of emergent plants from the lake bed following flooding. 6. The formation of floating mats comes with a cost and benefit to emergent plants. The cost is increased vulnerability to damage by water level fluctuations or wind–wave action, leading to reduced horizontal distribution. The benefit is that deep flooding is avoided, thus increasing vertical distribution. The net effect may be to lead to dominance of mat‐forming plants in low‐energy environments and non‐mat‐forming plants in high‐energy environments.     

The Role of Habitat and Herbivory on the Restoration of Tidal Freshwater Submerged Aquatic Vegetation Populations

Submerged aquatic vegetation (SAV) has declined precipitously throughout coastal areas and its reestablishment has long been an important objective of coastal management. We investigated restoration success of Vallisneria americana (wild celery) using seeds, seed pods, and whole shoot transplants at sites in the Chesapeake Bay in the United States where historical aerial photography has indicated that the species once grew. In addition, we evaluated habitat conditions and established herbivore exclosures to assess the impacts of water quality, sediment conditions, and grazers on planting success. Whole shoot transplants resulted in the most rapid cover of the bottom, but required greater planting effort. Direct dispersal of individual seeds was generally more successful than dispersal of intact seed pods, resulting in more rapid initial seedling growth. Overall, 100% bottom cover of whole shoot transplant plots could be reached in approximately 3 years, despite light attenuation coefficients (K_d) of 3.0 to 4.0. Transplants at shallow depths (<0.5 m) were able to rapidly grow and elongate to the surface at mid‐to‐low tidal heights. Transplants were successful in both muddy (8% organic) and sandy (<2%) substrates. Using mesh exclosures to protect the plants from herbivory was critical to restoration success. Although water quality and other habitat conditions are important for SAV growth and survival, restoration in the unvegetated areas studied here was limited by grazing of initial recruits. The establishment of protected founder colonies of sufficient size to withstand initial grazing pressures may be required to reestablish SAV in similar areas.