Maximum Depth Of Colonization Research Articles

Ecosystem changes in eight Danish lakes after zebra mussel invasion

AbstractInvasion of zebra mussels (Dreissena polymorpha) can have profound effects on lake ecosystems. We investigated the overall ecological effects after invasion and fast expansion of zebra mussels in eight lakes in Denmark belonging to the same watercourse systems but with different morphological and eutrophication characteristics. A few years after the invasion, most of the lakes had experienced profound changes in lake water quality, e.g. chlorophyll a had been reduced by 18–62% and Secchi depth had increased by 38–171%. Nutrient concentrations also decreased, total phosphorus to 37–64% and total nitrogen to 49–77% of pre-invasion concentrations. Seasonally, differences between both summer and winter phosphorus and nitrogen concentrations diminished, suggesting a change in internal nutrient cycling after the invasion. In most lakes, the abundance of submerged macrophytes rose substantially, and often coverage increased by a factor 10 or more, and the maximum colonisation depth increased by up to 5.4 m. Plant species numbers increased markedly in four of the six lakes with macrophyte data. The fish community changed towards higher dominance of potential piscivorous perch and an increase in their body size. The profound effects of zebra mussel invasion may affect lake management and how ecological quality should be interpreted.

Niche differentiation between a native and an invasive species of submersed macrophyte in a subtropical reservoir

Submersed macrophytes have important ecological roles but non-native invasive species may affect biodiversity and water uses. We investigated the native macrophyte Egeria najas and the invasive Hydrilla verticillata and measured their maximum colonization depth and its relationship with Secchi disk depth, their biomass along the depth gradient and their preferred depths of occurrence. The Itaipu Reservoir was monitored for seven years, during which maximum colonization depth and Secchi disk depth were measured. During a separate sampling, plants were collected to determine biomass along the depth gradient. Ancova showed that the maximum colonization depth of both species increased with increasing Secchi disk depth, but the maximum colonization depth of H. verticillata increased faster with increasing water transparency than did that of E. najas. Quadratic regression revealed that the biomass of each species peaks at intermediate depths. Hydrilla verticillata colonizes deeper regions than does E. najas. The patterns found in the present study can be explained by underwater light and, probably, wave disturbances. The preference of H. verticillata for deeper sites indicates that the ecological niches of the two macrophytes differ, and that H. verticillata has great potential to spread and accumulate biomass in reservoirs.

New Emphasis on Water Transparency as Socio-Ecological Indicator for Urban Water: Bridging Ecosystem Service Supply and Sustainable Ecosystem Health

The perspective on water transparency changed since the early days of limnology from being a physical parameter of optical water property to an ecological indicator tracking algal turbidity due to eutrophication or an overall success of sustained lake restoration in the late 60ies to 80ies. In modern cities, where ecosystems are commonly deteriorated by man-made modifications, water transparency offers a great opportunity to the public to raise socio-ecological consciousness concerning urban green-blue spaces. We thus re-emphasise water transparency as a key indicator of multi-functional value when assessing an oxbow lake of the riverine floodplain in Vienna, the Alte Donau. Our study covers the eutrophication from 1987-1994 due to the inclusion of the riverine landscape in the urban area, the following lake restoration with an ecosystem shift from a nutrient-rich, algal-turbid water body to a nutrient-poor, clear-water macrophyte controlled system and the impact of global warming in recent decades. We used light attenuation profiles to identify depth layers of specific ambient light requirements for photosynthetic domains (phytoplankton and submerged macrophytes), and to interpret Secchi measurements. Here we calculated the depth at 1% (minimum light requirements for phytoplankton growth as euphotic depth), 3% (minimum light requirements for macrophytes as maximum macrophyte colonization depth) and 12% (preferred light requirements for phytoplankton development) of surface ambient light. A Secchi disk water transparency of 1.5m (“lake bottom view”), judged as good water quality by human perception, refers to mesotrophic conditions with a maximum colonization depth for macrophytes exceeding the mean lake depth in Alte Donau. Water clarity required for sustained macrophyte growth, in particular for favouring bottom-dwelling Chara meadows instead of tall-growing Myriophyllum spicatum, is 3.5m Secchi depth and thus exceeds by far water clarity requested due to bathing aesthetics. Global warming, mirrored by an advanced warming in spring seems to favor significantly a higher yield of macrophytes mainly built up by Myriophyllum at the expense of the yield of algae. The prolongation of the summer period above 21°C, however, coincides with lowered Secchi transparency. Water visibility during the hot season thus seems to be slightly hampered against lake restoration efforts by global warming.

Spatiotemporal monitoring of hydrilla [Hydrilla verticillata (L. f.) Royle] to aid management actions

AbstractHydrilla is an invasive aquatic plant that has rapidly spread through many inland water bodies across the globe by outcompeting native aquatic plants. The negative impacts of hydrilla invasion have become a concern for water resource management authorities, power companies, and environmental scientists. The early detection of hydrilla infestation is very important to reduce the costs associated with control and removal efforts of this invasive species. Therefore, in this study, we aimed to develop a tool for rapid, frequent, and large-scale monitoring and predicting spatial extent of hydrilla habitat. This was achieved by integrating in situ and Landsat 8 Operational Land Imager satellite data for Lake J. Strom Thurmond, the largest US Army Corps of Engineers lake east of the Mississippi River, located on the border of Georgia and South Carolina border. The predictive model for presence of hydrilla incorporated radiometric and physical measurements, including remote-sensing reflectance, Secchi disk depth (SDD), light-attenuation coefficient (Kd), maximum depth of colonization (Zc), and percentage of light available through the water column (PLW). The model-predicted ideal habitat for hydrilla featured high SDD, Zc, and PLW values, low values of Kd. Monthly analyses based on satellite images showed that hydrilla starts growing in April, reaches peak coverage around October, begins retreating in the following months, and disappears in February. Analysis of physical and meteorological factors (i.e., water temperature, surface runoff, net inflow, precipitation) revealed that these parameters are closely associated with hydrilla extent. Management agencies can use these results not only to plan removal efforts but also to evaluate and adapt their current mitigation efforts.

HeLM: a macrophyte-based method for monitoring and assessment of Greek lakes

The Water Framework Directive (WFD) requires Member States to develop appropriate assessment methods for the classification of the ecological status of their surface waters. Mediterranean region has lagged behind in this task, so we propose here the first developed method for Greek lakes, Hellenic Lake Macrophyte (HeLM) assessment method. This method is based on two metrics, a modified trophic index and maximum colonization depth Cmax that quantify the degree of changes in lake macrophytic vegetation, as a response to eutrophication and general degradation pressures. The method was developed on the basis of a data set sampled from 272 monitoring transects in 16 Greek lakes. Sites from three lakes were selected as potential reference sites by using a screening process. Ecological quality ratios were calculated for each metric and for each lake, and ecological status class boundaries were defined. For the evaluation of effectiveness of the method, the correlations between individual metrics and final HeLM values and common pressure indicators, such as total phosphorus, chlorophyll a and Secchi depth, were tested and found highly significant and relatively strong. In addition, the ability of HeLM values and its individual metrics to distinguish between different macrophytic communities’ structure was checked using aquatic plant life-forms and found satisfactory. The HeLM method gave a reliable assessment of the macrophytic vegetation’s condition in Greek lakes and may constitute a useful tool for the classification of ecological status of other Mediterranean lakes.

Light requirements of water lobelia (Lobelia dortmanna L.)

Abstract Maximum depth of colonization (zC) and total area covered by a population of Lobelia dortmanna, as well as underwater light regime were studied in 25 soft water lobelia lakes in north-western Poland. Variations in underwater light conditions among the lakes were described by Secchi disc depths (zSD), and by attenuation coefficients of irradiance within photosynthetically active radiation range (Kd,PAR), and euphotic zone depths (zEU) derived from photometric measurements conducted twice a year (in midspring and midsummer) during the period 2014–2015. Maximum depth of colonization of water lobelia ranged from 0.1 to 2.2 m (median zC = 0.8 m; mean zC = 1.0 m). Nine lakes showed the relative coverage of the littoral zone (RCLZ) by L. dortmanna to be greater than the mean value, which was 4.8%. Studies showed that light requirements of water lobelia increase when the maximum depth of colonization also increases. This pattern could be partially related to the greater energy needs of deeper growing individuals due to enlarged seed production and their incubation, and for the creation of much heavier inflorescences. Assessment of the light requirements of L. dortmanna along the depth gradient indicates that relative irradiance (percentage of subsurface irradiance of PAR) should be at the level of: (i) 47–50% (annual total of quantum irradiance 3083–3280 mol m−2 yr−2) for plants growing within a depth range of 2.0–2.5 m; (ii) 44–47% (2886–3083 mol m−2yr−1) for plants growing within a depth range of 1.5–2.0 m; (iii) 41–44% (2690–2886 mol m−2yr−2) for plants growing within a depth range of 1.0–1.5 m; and (iv) 34–41% (2230–2690 mol m−1 yr−1) for those growing in the littoral zone at a depth of between 0.5 and 1.0 m. In average conditions in the Pomeranian lakes, the maximum depth of colonization by L. dortmanna accounts for approximately a third of the Secchi disc depth and a fifth of the depth of the euphotic zone with irradiance of PAR at zC equal to about 43% of subsurface irradiance. It has also been demonstrated that the light factor is a crucial one that limits the absolute maximum depth of lobelia population occurrence in Pomeranian lakes. The cleanest and most transparent lakes of this region have light attenuation coefficients (Kd,PAR) within the range of 0.35–0.42 m−1, which corresponds to the maximum colonization depths of 1.8–2.2 m.

Assessment of a simple hydroacoustic system for the mapping of macrophytes in extremely shallow and turbid lagoon

In this study, we focused on relatively inexpensive and commonly used fishermen class sonar in order to test its ability to detect stands of charophytes and other groups of submerged angiosperms in the shallow and turbid waters of the Curonian Lagoon. Based on the length of macrophytes, height thresholds (20 and 30cm) were established in order to distinguish charophytes from submerged angiosperms in obtained echograms. The echograms were visually analysed by two experts. We successfully discriminated 3 echofeatures (bare bottom, stands of charophytes and submerged angiosperms), whereas angiosperms from different morphology groups could not be distinguished. Below 1m depth, the stands of charophytes and submerged angiosperms were clearly distinguished, thus their maximum colonization depth could be delineated. The accuracy of the discrimination could be reduced by free-drifting mats of filamentous algae, resulting in the overestimation of charophytes. Our approach can be useful for the mapping of monospecific stands of submerged vegetation and could be an important additional tool for macrophyte monitoring and water quality assessment in shallow and turbid waterbodies.

Short‐term dynamics of submerged aquatic vegetation diversity and abundance in deep lakes

AbstractQuestionWhat are the short‐term trends of submerged aquatic vegetation (SAV) in deep lakes? Is it possible to measure significant changes in SAV distribution in deep lakes characterized by different trophic states and stable water physico‐chemical conditions using standard procedures over a 3‐yr period (2010–2013), as defined by the Water Framework Directive?LocationWe collected data from four deep lakes belonging to the Italian volcanic lake system (central Italy), which is one of the most important reserves of freshwater in Mediterranean Italy as well as one of the main hotspots of stonewort diversity in the Mediterranean area.MethodsMacrophytes were sampled along five transects in each lake in 2010 and 2013. Data on aquatic plant diversity, composition and abundance were collected from each 1‐m depth intervals down to the maximum colonization depth. Simultaneously, the water quality and climate were assessed by standard procedures. Data were analysed with PCA, ANOVA, PROTEST and rank abundance curves.ResultsThe sampling revealed a significant rearrangement of SAV. Increased species diversity in the first meters of depth (0–5 m) was due to the migration of mid‐depth species to lower depths. A reduction in the maximum growing depth of macrophytes was observed within the deepest stonewort communities (below 5 m). No significant changes emerged in water quality or climate.ConclusionsOver 3 yr, SAV displayed significant changes that appear to be independent of summer nutrient conditions and climate drivers. In particular, vascular and stonewort species underwent a distinct spatial rearrangement according to specific behaviours. The present data point to the need for more rigorous analyses of the intra‐annual dynamics of macrophytes in deep lakes. An influence of the significant increase in spring precipitation (which is in keeping with local climate variability) recorded in central Italy in the last decade cannot be ruled out a priori, nor can the possibility of stochastic fluctuations be excluded. All these considerations further highlight the critical role of aquatic plants as bioindicators.

Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water, UK

Summary Highly calcareous (marl) lakes are infrequent but important freshwater ecosystems, protected under the EU Habitats and Species Directive. Chara lakes have been considered resistant to eutrophication owing to the self‐stabilising properties of charophyte meadows. However, the opposite is suggested by the large‐scale biodiversity declines in marl lake taxa in Europe, and evidence of charophyte sensitivity to eutrophication. We combined contemporary, palaeolimnological and archival methods to investigate the eutrophication of Hawes Water, a shallow marl lake in north‐west England (U.K.). Changes in aquatic macrophyte and invertebrate communities were reconstructed through the analysis of historical macrophyte surveys and sedimentary plant and animal macrofossils in two dated sediment cores from the littoral and deep zones of the lake. In addition, chlorophyll and carotenoid pigments were analysed to track changes in primary production from benthic and pelagic areas. Substantial changes in macrophyte communities were detected over centennial timescales, suggesting high ecosystem sensitivity considering the presently moderate phosphorus concentrations in Hawes Water (mean annual total phosphorus 20 μg L−1). Two apparent periods of threshold‐like change were identified from the sediment record: (i) changes in cyanobacteria (aphanizophyll + myxoxanthophyll to canthaxanthin + zeaxanthin) and potentially in nutrient stoichiometry, reductions in the maximum macrophyte colonisation depth and water clarity, reduced charophyte and Potamogeton diversity, and increases in Nymphaeaceae; and (ii) severe reductions in light availability inferred from subdecadal doubling in phytoplankton abundance, substantial increases in Daphnia abundance and the extinction of charophytes from higher water depths. Further, change in both the littoral and deeper water has confined key marl lake taxa to smaller niches. In the littoral, increasing siltation and reed and Nymphaeaceae densities caused extinction of Littorella uniflora in the early 1900s and have reduced the evenness of Characeae with suspected imminent extinction of two highly localised Chara spp. In the deeper water, upslope creep of maximum colonisation depth has reduced habitat for intermediate‐depth marl lake taxa leading to the loss of four Potamogeton and one Chara species, and replacement of these taxa by Nuphar lutea. The large changes in macrophyte community composition and increased incidences of turbid water have reduced the distinctive and valued marl lake features of Hawes Water, indicating that marl lakes can, as a habitat type, be highly sensitive to eutrophication. The persistence of abundant generalist macrophyte species at considerable water depth may be a feature of high‐alkalinity lakes in clearwater, macrophyte‐dominated states, but is a distinct eutrophication response in marl lakes rather than an indication of resistance to eutrophication.

Assessing the status of marl lakes under the European Union Water Framework Directive – insights from contemporary and palaeolimnological studies of three English lakes

Methods for ecological status assessment of high-alkalinity lakes under the Water Framework Directive (WFD) differ between continental Europe and the UK. In the UK, marl lakes, i.e. carbonate-precipitating lakes, are placed in a separate category in which metrics account for the naturally low phosphorus content and sensitive macrophyte communities found therein. If this separation is appropriate, the UK method, which applies a correction factor for stricter assessment of marl lakes, should assess ecological status more effectively. Using contemporary macrophyte and chlorophyll-a data, this supposition was assessed for three English lakes by calculating ecological status with and without the correction factor. The veracity of the results was assessed using palaeolimnological and historical surveys. Further, the potential of macrophyte maximum colonisation depth (Cmax) as an additional metric was investigated. For two lakes, the status suggested by historical-palaeolimnological data was poorer (Moder- ate) than that suggested by WFD classification, both with and without the correction factor (Good and/or High). This has important implications as restoration is required only for water bodies classed below Good. For the third lake, for which high chlorophyll-a resulted in Poor status in agreement with historical-palaeolimnological data, the discrepancy between the macrophyte metrics with and without the correction factor was considerable (Poor, Moderate, respectively). The results suggest that a separate marl lake category is warranted and may be advisable for other Member States in continental Europe. Importantly, status was higher than recommended for the two lakes close to the Good/Moderate boundary even with the correction factor. This leads us to recommend more studies be undertaken on eutrophication responses of marl lakes. In lakes where macrophyte species composition varies with water depth, eutrophication is apparent through reductions in Cmax especially of Characeae and Potamogetonaceae. Cmax is simple to measure and may much improve the reliability of marl lake assessment.

The effect of lake morphology on aquatic vegetation development and changes under the influence of eutrophication

Data on aquatic and emergent vegetation, morphology and water quality from 274 Polish lowland lakes surveyed in the years 1996–2009 were used to validate the preliminary typology of Polish lakes based on macrophytes and to indicate the environmental parameters which most significantly determine the vegetation patterns in lakes under various morphological conditions. In highly alkaline lowland lakes representing non-disturbed conditions the key determinants influencing the vegetation patterns were mean depth and the shape of the littoral. Three morphological lake types were distinguished: shallow (<3.5m), deep, and additionally, within the latter, deep ribbon-shaped, with a clearly elongated base and steep bed slopes. The lake types varied in their vegetation patterns developed under non-disturbed conditions. In the shallow lakes, the share of the phytolittoral in the total lake area (%phytol) was the highest (40–100%, 72.3% on average) and the maximum colonisation depth (Cmax) the lowest (3.2m as the maximum) compared to the lakes from both deep types. In the ribbon-shaped deep lakes, %phytol and the plant coverage (%cover) were the lowest, the proportion of submerged vegetation was extraordinarily high (over 90%) and the emergent vegetation was extremely sparsely developed (<6%) compared to the lakes of the two other types.The alterations of aquatic vegetation resulting from the eutrophication process in distinguished morphological lake types were explored. Within the macrophyte variables tested, three groups of indicators were distinguished: (a) metrics performing best in selected lake types, i.e. the type-specific indicators (abundance metrics, %Pota), (b) metrics performing equally well in all the lake types, considered as the universal indicators (e.g. S_Chara, %Subm and %Emerg) and (c) metrics performing poorly in all the lake types, with generally limited applicability (most of the metrics on syntaxonomic richness). In the shallow lakes, %cover and %phytol performed notably better than in deep lakes, whereas Cmax worked best in deep lakes and showed the strongest response in the deep regular-shaped lakes. Moreover, in deep regular-shaped lakes the number of communities of stoneworts and submerged plants (S_Chara and S_Subm), and in deep ribbon-shaped lakes the proportion of area inhabited by vascular plant communities (%Pota) performed exceptionally better than in the other two lake types. The most universal metrics, performing equally well in all the lake types, were the proportions of submerged (%Subm) and emergent (%Emerg) vegetation in the total phytolittoral area.

Chemical and Biological Responses of Marl Lakes to Eutrophication

Eutrophication remains one of the foremost environmental issues threatening the quality of surface waters yet comparatively little is known of the timing, magnitude and characteristics of nutrient-related changes in highly calcareous (marl) lakes. This review focuses on marl lake ecology and chemistry, their known responses to eutrophication, and also highlights questions that remain unanswered.In good condition, marl lakes support a diversity of macrophytes, especially Characeae and Potamogetonaceae, which can grow to considerable depth. High water transparency and low phosphorus and phytoplankton concentrations are facilitated by the coprecipitation of marl and phosphorus. Although large amounts of phosphorus can be thus removed, buffering against eutrophication, macrophyte communities can undergo significant change under rather low nutrient concentrations. Maximum colonisation depth declines and tolerant species replace sensitive species, with losses particularly among charophytes. Marl lakes are therefore ecologically highly sensitive.The effects of coprecipitation on long-term burial of phosphorus are contested. Several palaeolimnological studies have identified iron complexes as more important than calcite, as chemical conditions in the sediment may promote either calcite dissolution or calcite-bound phosphorus exchange, or possibly both. Some marl lakes have been shown to have phosphorus concentrations which, compared with other lake types, are higher than expected in winter and lower in summer. The phosphorus binding capacity of marl sediment has not to our knowledge been adequately researched.Marl precipitation may be inhibited by high phosphate or organic matter concentrations in the water, or when biological communities effecting precipitation (picoplankton, charophytes, epiphytes) are disturbed. Highly impacted marl lakes having low species diversity and lacking precipitation may be misidentified as eutrophic, high-alkalinity lakes. More studies addressing the interaction between external loading, phosphorus cycling and marl precipitation in relation to biological communities are required to assess to what extent marl lakes can buffer eutrophication, and what factors contribute to disturbed marl precipitation.

Optical properties of highly turbid shallow lakes with contrasting turbidity origins: The ecological and water management implications

A comprehensive optical study of three highly turbid shallow lakes was presented. The lakes contained very high concentrations of optically active substances [OASs] with clear differences in total suspended solid [TSS] composition among them. Lakes presented elevated values of total absorption [at(λ)] and scattering coefficients [b(λ)], which translated into extremely high light attenuation coefficients [Kd(PAR)]. Differences among lakes in the estimation of Kd(PAR), using two typical estimators of light penetration (i.e., nephelometric turbidity [Tn] and Secchi disk [ZSD]), were analysed. Kirk's optical model was used to model Kd(PAR) using inherent optical properties [IOPs]. Modelled values of Kd(PAR) agreed very well with those measured (R2 = 0.95). In addition, optical properties and Kirk's model were used to determine water quality targets for restoring submerged aquatic vegetation [SAV]. Based on a minimum light requirement for SAV of 10%, results showed that only an integrative remediation action, considering substantial reduction of TSS and Chl a (95%), and CDOM (50%), must be contemplated to improve maximum colonization depth for SAV to values higher than 0.7 m. On the other hand, phytoplankton absorptive characteristics were also studied. In these lakes, phytoplankton showed different responses to the nature of light competition. Some of the variation in specific phytoplankton absorption [aph∗(λ)] was explained by differences in the ratio between unpigmented particulate absorption and phytoplankton absorption (up to R2 = 0.48 for the blue band). Hydrologic optical results were discussed in terms of ecological and management implications.

Submerged aquatic vegetation in the Bay of Quinte: Response to decreased phosphorous loading and Zebra Mussel invasion

Originally mesotrophic, the Bay of Quinte ecosystem has experienced eutrophication since the 1940s, which resulted in the decline of once-lush submerged aquatic vegetation (SAV) beds in the upper bay by the mid-1960s. Since 1972, twelve SAV surveys have been conducted along ten index transects, recording:% cover, distance SAV beds extended from shore (extent), maximum depth of colonization (Zc), species composition, and, in later years, wet plant biomass. Offshore secchi depth and ϵpar, (the vertical light extinction rate (m−1) for photosynthetically active radiation), were also recorded either weekly or bi-weekly during the growing season since 1972. During this time, two major changes occurred within the bay: the reduction in point-source phosphorus (P-control) loadings in 1978 and the 1993 invasion by Dreissenid Mussel. SAV response to these changes varied temporally and spatially, with the shallow upper bay showing the greatest response, particularly after Zebra Mussels establishment. In the upper bay, mean secchi depth increased by 8% from 1.2 m prior to P-control (pre-P), to 1.3 m after P-control (post-P) and further increased by 46% to 1.9 m after Dreissena establishment (post-D). Upper bay SAV responded to these invasive species with increases in the means of three variables: Zc from 1.6 to 3.5 m, extent from 114 m to 417 m and wet biomass from 50 g m−2 to 962 g m−2. SAV in the middle and lower bays were in better condition in 1972, with pre-P cover in excess of 50% and Zc of 2.6 and 3.7 m, respectively. SAV cover did increase in the post-D (1994 to 2007) period by approximately 25% and Zc increased to 3.7 and 6.5 m, but the narrow fringing strip of shallower water along the shore in these two deeper bays limited substantial increases in bed extent. Both water clarity and basin morphometry strongly influenced SAV distribution and abundance within the Bay of Quinte.

Maximum growing depth of submerged macrophytes in European lakes

Submerged macrophytes are important elements for the structure and functioning of lake ecosystems. In this study, we used chemical and maximum colonisation depth (C_max) data from 12 European countries in order to investigate how suitable C_max may describe the impact by eutrophication. The analyses include data from 757 lakes and 919 lake years covering oligotrophic to eutrophic lakes. Overall, C_max was closely related to Secchi depth (R 2 = 0.58) and less closely to chlorophyll a (R 2 = 0.31), TP (R 2 = 0.31) and total nitrogen, TN (R 2 = 0.24). The low coefficients of determination between C_max and nutrient concentrations suggest that other response factors than nutrient-phytoplankton-light conditions are important for C_max and that it will be difficult to establish strong relationships between external nutrient loading and C_max. Yearly monitoring for 13–16 years in eight Danish lakes showed considerable year-to-year variability in C_max, which for the individual lakes only related weakly to changes in Secchi depth. The use of C_max as an eutrophication indicator is especially relevant in not very shallow lakes (maximum depth >4–5 m), not too turbid lakes (C_max >1 m) and not very humic lakes (colour <60 mg Pt/l).

Submerged macrophytes as indicators of the ecological quality of lakes

Summary1. We analysed submerged macrophyte communities from 300 Danish lakes to determine the efficacy of different species, maximum colonisation depth (Cmax) of plants as well as coverage and plant volume inhabited (PVI) as indicators of eutrophication.2. Most species occurred at a wide range of phosphorus and chlorophyll a (Chla) concentrations, but some species of isoetids (Lobelia, Isoëtes) and Potamogeton (Potamogeton gramineus, Potamogeton alpinus and Potamogeton filiformis) were mainly found at low nutrient concentrations and hence may be considered as indicators of nutrient poor conditions. However, species typically found in nutrient‐rich conditions, such as Elodea canadensis and Potamogeton pectinatus, were also found at total phosphorus (TP) &lt;0.02 mg P L−1 and Chla &lt;5 μg L−1 and therefore cannot be considered as reliable indicators of eutrophic conditions.3. Submerged macrophyte coverage, PVI and the Cmax were negatively correlated with TP and Chla. However, variability among lakes was high and no clear thresholds were observed. At TP between 0.03 and 0.07 mg P L−1 plant coverage in shallow lakes ranged from nearly 0 to 100%, whilst at concentrations between 0.10 and 0.20 mg P L−1 only 29% of the lakes had coverage &gt;10%. Cmax was found to be a useful indicator only in deep lakes with unvegetated areas in the deeper part, whereas the use of coverage was restricted to shallow lakes or shallow areas of deep lakes.4. Overall, submerged macrophytes responded clearly to eutrophication, but the metrics investigated here showed no well‐defined thresholds. We developed a simple index based on species richness, presence of indicator species, coverage and Cmax, which might be used to track major changes in macrophyte communities and for lake classification.

An evaluation of methods for sampling macrophyte maximum colonisation depth in Loch Leven, Scotland

The maximum colonisation depth (MCD) of macrophytes in lakes is considered a robust ecological indicator of system health. The accuracy of five boat-based survey methods (double-headed rake, Ekman grab, bathyscope, hydroacoustics, and underwater photography) was evaluated along depth-transects within a shallow eutrophic loch (Loch Leven, Scotland, UK) in 2006. Data from an earlier field campaign conducted in 1993 (33 transects) were used to assess sample frequency and error associated with the use of the double-headed rake method. The double-headed rake consistently sampled more species than the Ekman grab or bathyscope, probably as a result of its ability to sample relatively larger areas of the lake bed in comparison to the other methods. The bathyscope method underestimated the MCD in comparison to all other methods. This appeared to be because the turbidity of the water-column restricted the depth of view when using the bathyscope. The double-headed rake and underwater photography methods were in good agreement, with maximum observed MCD estimates of 5.0 m and 4.5 m water depth, respectively (2006). Using the 1993 data (MCD observations taken from 33 selected transects), we estimated that over 5 transects were required to ensure a MCD estimate greater than the 75th percentile from the distribution of all 33 colonisation depths, more than 15 replicate transects were required to ensure an estimate greater than the 90th percentile, and more than 20 replicate transects were required to ensure an estimate greater than the 95th percentile.

Factors affecting the maximum depth of colonization by submersed macrophytes in Florida lakes

In 32 Florida lakes, Secchi depth (SD), light attenuation coefficient measured with a light meter, plant and sediment type, and slope were examined with respect to the maximum depth of plant colonization (MDC). The MDC was shown to be significantly related to light through measurements taken by SD (R2 = 0.46; p < 0.05) and a light meter (R2 = 0.41; p < 0.05). While both light measurements can be used to estimate MDC, SD accounted for more variance in MDC than light attenuation coefficients. Plant type, sediment type and slope did not account for more variance in MDC than light measurements for these Florida lakes. Additional unpublished data from 187 Florida Lakes (Florida LAKEWATCH, 279 lake-years of data) also showed a significant positive relationship between SD and MDC (R2 = 0.68; p < 0.05). The best fit MDC-SD regression line in meters was: log (MDC) = 0.66 log (SD) + 0.31. A maximum MDC line relating MDC to SD in meters was also calculated and was found to be equal to: log (max MDC) = 0.53 log (SD) + 0.59. The maximum MDC line describes light limitation when the MDC response falls on or near the response curve, and when MDC values fall below the line, some other factor likely limits colonization of macrophytes.

Effect of reservoir drawdown on biomass of three species of aquatic macrophytes in a large sub-tropical reservoir (Itaipu, Brazil)

The biomass and maximum depth of colonization (Zmax) of Egeria najas Planchon (submerged), and the biomass and area covered by a stand of Eichhornia crassipes (Mart.) Solms and Salvinia herzogii Raddi (floating species) were assessed to measure the effects of a 5 m drawdown in water level of the Itaipu Reservoir (Brazil—Paraguay), which lasted about 3 months. The frequency of occurrence of the two first species, and of Salvinia spp was also assessed in the main arms of the reservoir. A conspicuous decrease of E. najas biomass was observed and this attribute did not reach the previous values even 10 months after the water level had returned to normal. Rapid growth of free-floating species (surface t doub = 2.9 and 3.2 days for total biomass of S. herzogii and E. crassipes, respectively, and 2.3 days for surface area covered by both species) was recorded immediately after the water level returned to normal. This fast growth was related to phosphorus increases in water. A clear succession was observed over a period of 103 days, during which S. herzogii was slowly substituted by E. crassipes. The effects of water level drawdown were also observed on E. najas frequency, given that this species occurred in 38% of the stands investigated before the water drawdown, but in only 9 and 6% of stands following 1 and 10 months of water level recovery, respectively. Water drawdown did not affect the frequency of occurrence of the floating species, which remained approximately constant, and lower than 15% in several of the reservoir arms investigated.

Biomass distribution, growth and decay of Egeria densa in a tropical high-mountain reservoir (NEUSA, Colombia)

In tropical settings, freshwater macrophytes can reach abundant biomasses. The combination of high temperatures, high irradiances and a continuous growing season are commonly held responsible for such prolific growth. Most research in tropical systems has been done in low altitudes. High-altitude systems, though, are common settings for macrophytes in the tropics, and constitute a unique environment as temperatures can reach values comparable with sub-tropical winter ranges on a daily basis. We sought to study the biomass dynamics, and distribution of Egeria densa P., a dominant submerged weed in Neusa Reservoir (Colombian Andes, 2970 m.a.s.l.). The specific objectives were: (1) to assess the spatial distribution of standing biomass of E. densa in relation to some characteristics of the littoral zone, (2) to study its growth and decay rates as a function of water depth and (3) to compare standing biomass, growth and decay rates in Neusa with those found for other laboratory and natural systems. E. densa occupied a continuous fringe along the littoral and was present on slopes up to 40%. It was found down to 7 m in depth. Average biomass was 522 g m −2, was highest at intermediate depths and increased with slope and proximity to water effluents. Maximum depth of colonization and maximum biomass were underestimated when using available models based on Secchi depth. Maximum biomass was the highest macrophyte biomass reported in the literature while mean biomass was among the highest. Relative growth rate and elongation rates were negatively affected by depth. RGR as well as decomposition rate were low in comparison with estimates for E. densa in other systems. When analyzing available data from other studies and the present one, mean air temperature was better than latitude at predicting mean biomass. We propose that the high values of standing biomass in Neusa result from the combination of low decay rates and low but continuous growth throughout the year, conditions only possible in this kind of environment. This study provides evidence of the distinctiveness of high-altitude tropical macrophyte stands in terms of distribution, biomass dynamics and their regulating factors.