Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration

Lake Pamvotis is a moderately sized (22 km²) shallow (z _avg=4 m) lake with a polymictic stratification regime located in northwest Greece. The lake has undergone cultural eutrophication over the past 40 years and is currently eutrophic (annual averages of FRP=0.07 mg P l^-1, TP=0.11 mg P l^-1, NH₄ ⁺=0.25 mg N l^-1, NO₃ ^–=0.56 mg N l^-1). FRP and NH₄ ⁺ levels are correlated to external loading from streams during the winter and spring, and to internal loading during multi-day periods of summer stratification. Algal blooms occurred in summer (July–August green algae, August–September blue-green algae), autumn (October blue-green algae and diatoms), and winter (February diatoms), but not in the spring (March–June). The phytoplankton underwent brief periods of N- and P-limitation, though persistent low transparency (secchi depth of 60–80 cm) also suggests periods of light limitation. Rotifers counts were highest from mid-summer to early autumn whereas copepods were high in the spring and cladocerans were low in the summer. Removal of industrial and sewage point sources a decade ago resulted in a decrease in FRP. A phosphorus mass balance identified further reductions in external loading from the predominately agricultural catchment will decrease FRP levels further. The commercial fishery and lake hatchery also provides opportunities to control algal biomass through biomanipulation measures.     

The effects of Daphnia on nutrient stoichiometry and filamentous cyanobacteria: a mesocosm experiment in a eutrophic lake

1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P‐rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north‐western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N‐fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P. 2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH₄, –NH₄) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment. 3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH₄ had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients. 4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH₄). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures. 5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.     

Seasonal variation in the water quality and chlorophyll a of semi-intensive shrimp ponds in a subtropical environment

Twelve water quality variables were measured at different intervals (biweekly and twice a week) in two ponds during two consecutive production cycles in a semi-intensive shrimp farm on the Northwest coast of Mexico. The average harvest during dry season (March–July, a 95 days period) was 1822 kg ha^-1, with an average size of 16.0 g; while the harvest in the rainy season (August–January, a 165-day period) was 2125 kg ha^-1, with an average size of 11.9 g. In the rainy season, dissolved oxygen concentration tended to decrease as the feeding rates and shrimp and phytoplankton biomass were increasing until harvest. During both culture cycles, mean values of temperature, salinity and ammonium in ponds were not significantly different from those in inlet water, whereas pH levels in ponds were higher than in incoming water.Nitrogen nutrients, pH and particulate organic matter levels in ponds and inlet water did not exhibit seasonal differences; the better yield and feed conversion obtained during rainy season reflect the fact that temperature and mean chlorophyll a were higher during the rainy than in the dry season; salinity and total suspended solids were lower during the dry than in the rainy season. The mean levels of the water quality variables in the two ponds were not significantly different in both grow out cycles. Therefore, the differences in productivity between ponds cannot be explained by the water quality.     

Long‐term phytoplankton community changes in a deep subalpine lake: responses to nutrient availability and climatic fluctuations

1. In natural lakes, modifications in the species composition and abundance of phytoplankton communities may ultimately be responses to changes in nutrient availability and climatic fluctuations. Phytoplankton and associated environmental factors were collected at monthly intervals from the beginning of the 1990s to 2007 in the large subalpine Lake Garda (z_max = 350 m, V = 49 × 10⁹ m³). In this study period, the lake showed a slight and continuous increase of total phosphorus (TP) in the water column, up to concentrations of 18–20 μg P L^?1. This increase represented the last stage of a long‐term process of enrichment documented since the 1970s, when concentrations of TP were below or around 10 μg P L^?1. 2. At the community level, annual phytoplankton cycles underwent a unidirectional and slow shift mainly due to changes in the species more affected by the nutrient enrichment of the lake. After a first and long period of dominance by conjugatophytes (Mougeotia) and diatoms (Fragilaria), phytoplankton biomass in recent years was sustained by cyanobacteria (Planktothrix). Other important modifications in the development of phytoplankton were superimposed on this pattern due to the effects of annual climate fluctuations principally mediated by the deep mixing events at spring overturn and, secondarily, by temperature and thermal stability of the water column during the growing season. 3. Interannual variations in the stability and temperature of the water column appeared to influence the development of a few subdominant flagellates (dinophytes and cryptophytes). Nevertheless, the major impact of climate on phytoplankton was indirect, and mediated through the effects of winter climatic conditions on deep mixing dynamics. Winter climatic fluctuations proved to be a key element in a linked chain of causal factors including cooling of hypolimnetic waters, deep vertical mixing and epilimnetic nutrient replenishment. The process of fertilisation was measurable both for TP and dissolved inorganic nitrogen, although only the first had a large effect, reinforcing the seasonal growth of a few dominant groups. The degree of nutrient replenishment further increased the spring development of large diatoms and the increase of Planktothrix in summer and autumn. 4. Currently, changes in nutrient concentrations have the greatest effect on the phytoplankton community, while direct effects due to the interannual variations in the thermal regime are of secondary importance compared with the indirect effects mediated through deep water mixing and spring fertilisation. Overall, the results demonstrate that the consequences of climatic fluctuations and climate warming on phytoplankton communities need to be studied at different levels of complexity and integration, from the direct effects of temperature and thermal regime, to the indirect effects mediated by the physiographic characteristics of water bodies.     

Stoichiometric and growth responses of a freshwater filamentous green alga to varying nutrient supplies: slow and steady wins the race

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Plant diversity in a changing world: status,trends, and conservation needs

The conservation of plants has not generated the sense of urgency—or the funding—that drives the conservation of animals, although plants are far more important for us. There are an estimated 500,000 species of land plants (angiosperms, gymnosperms, ferns, lycophytes, and bryophytes), with diversity strongly concentrated in the humid tropics. Many species are still unknown to science. Perhaps a third of all land plants are at risk of extinction, including many that are undescribed, or are described but otherwise data deficient. There have been few known global extinctions so far, but many additional species have not been recorded recently and may be extinct. Although only a minority of plant species have a specific human use, many more play important roles in natural ecosystems and the services they provide, and rare species are more likely to have unusual traits that could be useful in the future. The major threats to plant diversity include habitat loss, fragmentation, and degradation, overexploitation, invasive species, pollution, and anthropogenic climate change. Conservation of plant diversity is a massive task if viewed globally, but the combination of a well-designed and well-managed protected area system and ex situ gap-filling and back-up should work anywhere. The most urgent needs are for the completion of the global botanical inventory and an assessment of the conservation status of the 94% of plant species not yet evaluated, so that both in and ex situ conservation can be targeted efficiently. Globally, the biggest conservation gap is in the hyperdiverse lowland tropics and this is where attention needs to be focused.     

Use of diatom indices to categorise impacts on and recovery of a floodplain system in South Africa

The trophic status of the Ramsar-accredited Nyl River floodplain, which is stressed by sewage treatment effluents, and its ability to restore normal conditions, were assessed in 2014–2015 using diatoms as biological indicators. The Trophic Diatom Index, Specific Pollution Sensitivity Index and the Generic Diatom Index were used to characterise water quality under high and low flow conditions in 2014. An additional survey was conducted in May 2015, following an accidental sulphuric acid spill above the Sewage Treatment Works (STW). Significant linear correlations were identified between physico-chemical parameters and the indices, as well as between the three indices themselves, validating their use to infer water quality. Water quality deteriorated from the source of the Klein Nyl River, especially at the STW, but improved within the Nylsvley Nature Reserve wetland sampling site (NYL).     

The response of periphyton and submerged macrophytes to nitrogen and phosphorus loading in shallow warm lakes: a mesocosm experiment

1. Recent experimental and field studies on temperate shallow lakes indicate that nitrogen may play a greater role in their functioning than previously thought. Several studies document that abundance and richness of submerged macrophytes, both central in shallow lake ecology, may decrease with increasing nitrogen loading, especially at high phosphorus levels. However, the role of nitrogen in warm lakes with fluctuating water regimes remains to be described in detail. 2. The effect of increasing nitrate and phosphate concentrations on submerged macrophyte growth was examined in a 3‐month mesocosm experiment conducted in summer in a shallow freshwater lake on the north western coast of Turkey with a Mediterranean climate. Twenty four field mesocosms, open to the sediment and atmosphere, were stocked with Myriophyllum spicatum shoots and small cyprinid fish. Three nitrate loadings in combination with two phosphate loadings were applied in a fourfold replicated design. 3. Mean ± SD nutrient concentrations maintained throughout the experiment were 0.55 ± 0.17, 2.2 ± 0.97, 9.2 ± 5.45 mg L^?1 total nitrogen and 55 ± 19.2, 73 ± 22.9 μg L^?1 total phosphorus. Mean periphyton biomass increased with increasing nutrient concentrations and peaked at the highest nitrogen and phosphorus loadings, while the mean phytoplankton biomass remained relatively low in all treatments. 4. Percent volume inhabited (% PVI) by macrophytes throughout the experiment and total macrophyte biomass at the end of the experiment did not differ among treatments. In addition to stocked M. spicatum, Ceratophyllum demersum and Potamogeton crispus appeared in the majority of the mesocosms. The plants grew continuously up to 50% PVI throughout the experiment and remained resilient to shading provided by periphyton and phytoplankton. 5. The mean summer air temperature in 2007 was 2.2 °C higher than the average of the last 32 years, which resulted in a water level decrease of 0.3 m in the mesocosms over three months. This might have counteracted the shading of submerged macrophytes provided by phytoplankton and periphyton. The results of the experiment are consistent with observations of higher macrophyte resilience to nutrient loading in Mediterranean lakes compared with northern temperate lakes.     

Nutrient concentrations in Nyanza Gulf, Lake Victoria, Kenya: light limits algal demand and abundance

Nyanza Gulf is a large shallow embayment connected to Lake Victoria by the deep narrow Rusinga Channel. Between December 2000 and May 2002 the gradient of nutrients along the axis of Nyanza gulf into the main lake was determined. Nutrient concentrations in the gulf were found to be different from those in the main lake with phosphorus fractions, SRP and TP being significantly higher in the main lake than in the gulf. Well oxygenated conditions in the gulf keep the PO₄–P strongly bound to mineral particles whereas in the main lake, where deeper depths allows for development of anoxia, it is released into solution. The low TN:TP molar ratio in the gulf and in the main lake indicates many algae may be N-limited and heterocystous N-fixing cyanobacteria may be favored. However, the high mineral turbidity in the gulf reduces light availability and hence limits algal abundance resulting in easily measure concentrations of mineral nutrients and in particular high levels of dissolved reactive silica. The gulf is a net source of dissolved silica and total nitrogen to the main lake while the main lake is potentially a source of P to the gulf depending on interchange through Rusinga Channel.     

Daily cycles of dissolved oxygen and nutrient content in a shallow fishpond: The impact of water renewal

This study was conducted in a shallow fishpond located on the eastern side of Arcachon Bay, France.Dissolved oxygen, pH and nutrient content (NH₄, NO₂, NO₃ and PO₄) showed pronounced diel variations.These phenomena seemed to be directly controlled by biological processes. Microbially mediated degradation of organic matter constitutes the main source of nutrients in the water column and primary producers consume the major part of these products. These processes are closely coupled to the light-dark cycle.Man-induced water renewal caused low modifications in nutrient concentration and did not alter its temporal dynamics.     

Epiphytic bromeliads in a changing world: the effect of elevated CO2 and varying water supply on growth and nutrient relations

• Global climate change is likely to impact all plant life. Vascular epiphytes represent a life form that may be affected more than any other by possible changes in precipitation leading to water shortage, but negative effects of drought may be mitigated through increasing levels of atmospheric CO₂.
• We studied the response of three epiphytic Aechmea species to different CO₂ and watering levels in a full‐factorial climate chamber study over 100 days. All species use crassulacean acid metabolism (CAM). Response variables were relative growth rate (RGR), nocturnal acidification and foliar nutrient levels (N, P, K, Mg).
• Both elevated CO₂ and increased water supply stimulated RGR, but the interaction of the two factors was not significant. Nocturnal acidification was not affected by these factors, indicating that the increase in growth in these CAM species was due to higher assimilation in the light. Mass‐based foliar nutrient contents were consistently lower under elevated CO₂, but most differences disappeared when expressed on an area basis.
• Compared to previous studies with epiphytes, in which doubling of CO₂ increased RGR, on average, by only 14%, these Aechmea species showed a relatively strong growth stimulation of up to +61%. Consistent with earlier findings with other bromeliads, elevated CO₂ did not mitigate the effect of water shortage.

     

Nutrient addition effects on chlorophyll a,phytoplankton biomass,and heterocyte formation in Lake Erie’s central basin during 2014–2017: Insights into diazotrophic blooms in high nitrogen water

1. Phosphorus (P) usually is the primary limiting nutrient of phytoplankton biomass, but attention towards nitrogen (N) and trace nutrients, such as iron (Fe), has surfaced. Additionally, N-fixing cyanobacterial blooms have been documented to occur in N-rich, P-poor waters, which is counterintuitive from the paradigm that low N and high P promotes blooms. For example, Lake Erie's central basin has Dolichospermum blooms when nitrate concentrations are high, which raises questions about which nutrient(s) are selecting for Dolichospermum over other phytoplankton and why an N-fixer is present in high N waters?
2. We conducted a 4-year (2014–2017) study in Lake Erie's central basin to determine which nutrient (P, N, or trace nutrients such as Fe, molybdenum [Mo], and boron [B]) constrained chlorophyll concentration, phytoplankton biovolume, and nitrate assimilation using nutrient enrichment bioassays. The enriched lake water was incubated in 1-L bottles in a growth chamber programmed at light and temperatures of in situ conditions for 4–7 days. We also quantified heterocytes when N-fixing cyanobacteria were present.
3. Compared to the non-enriched control, the P-enriched (+P) treatment had significantly higher chlorophyll and phytoplankton biovolume in c. 75% of experiments. Combination enrichments of P with ammonium-N, nitrate-N, Fe, Mo, and B were compared to the +P treatment to determine secondary limitations. +P and ammonium-N and +P nitrate-N resulted in higher chlorophyll in 50% of experiments but higher phytoplankton biovolume in only 25% of experiments. These results show that P was the primary limiting nutrient, but there were times when N was secondarily limiting.
4. Chlorophyll concentration indicated N secondary limitation in half of the experiments, but biovolume indicated only N secondary limitation in 25% of the experiments. To make robust conclusions from nutrient enrichment bioassays, both chlorophyll and phytoplankton biovolume should be measured.
5. The secondary effects of Fe, Mo, and B on chlorophyll were low (<26% of experiments), and no secondary effects were observed on phytoplankton biovolume and nitrate assimilation. However, +P and Fe resulted in more chlorophyll than +P in experiments conducted during Dolichospermum blooms, and +P and B significantly increased the number of heterocytes in Dolichospermum. These results indicate that low Fe availability might select for Dolichospermum, and low B constrains heterocyte formation in the central basin of Lake Erie. Furthermore, these results could apply to other lakes with high N and low P where diazotrophic cyanobacterial blooms occur.

     

Large herbivore conservation in a changing world: Surface water provision and adaptability allow wildebeest to persist after collapse of long‐range movements

Large herbivores, particularly wide‐ranging species, are extensively impacted by land use transformation and other anthropogenic barriers to movement. The adaptability of a species is, therefore, crucial to determining whether populations can persist in ever smaller subsets of their historical home ranges. Access to water, by drinking or from forage moisture, is an essential requirement, and surface water provision is thus a long‐established, although controversial, conservation practice. In the arid Kgalagadi Transfrontier Park (KTP), South Africa, surface water provision in the 1930s facilitated the establishment of a sedentary wildebeest (Connochaetes taurinus) population in a region historically accessed only in the wet season, via now collapsed long‐distance movements. Here, we investigate the behaviour and diet of this wildebeest population, and how these relate to water in the landscape, to better understand the process of transitioning from a mobile to sedentary population. Data from 26 monthly surveys reveal that wildebeest distributions are shaped by water availability and salinity, shade, forage, season and possibly predator detectability. Areas with saline or no water are used predominantly in the wet season when forage moisture is high. Wet season movements beyond the study area mean the timing of wildebeest grazing in these regions matches historical timing. Grass utilization field data suggest that the KTP grazer population experiences forage deficits during the dry season, when ~80% of grass tufts are grazed and C:N and crude protein levels decline. Nonetheless, dung isotope data show that wildebeest meet their crude protein intake requirements during the dry season, likely by consuming unprecedentedly high levels of browse (>33%). While restoring the full historical range and movements of most large herbivore populations is not possible, these findings highlight that understanding the behavioural and dietary adaptability of a species can augment ‘next best’ efforts to conserve viable populations while home ranges contract.     

Modelling algal growth and transport in rivers: a comparison of time series analysis,dynamic mass balance and neural network techniques

Algae present considerable problems for river qualitymanagers and water suppliers and methods to predicttheir behaviour, growth and transport can assist inoperational management. Alternative techniques existfor predicting algal response and three approacheshave been compared and applied to data from six sitesalong the River Thames. These techniques include timeseries analysis, dynamic mass balance and growthequations and neural network approaches. It is shownthat neural network techniques offer a new approachrequiring less intuitive knowledge but predictivecapability is not improved greatly compared to otherapproaches. Neural networks enable models to bedeveloped along all six reaches of the RiverThames.     

Fire‐induced tree mortality in a neotropical forest: the roles of bark traits,tree size,wood density and fire behavior

Large‐scale wildfires are expected to accelerate forest dieback in Amazônia, but the fire vulnerability of tree species remains uncertain, in part due to the lack of studies relating fire‐induced mortality to both fire behavior and plant traits. To address this gap, we established two sets of experiments in southern Amazonia. First, we tested which bark traits best predict heat transfer rates (R) through bark during experimental bole heating. Second, using data from a large‐scale fire experiment, we tested the effects of tree wood density (WD), size, and estimated R (inverse of cambium insulation) on tree mortality after one to five fires. In the first experiment, bark thickness explained 82% of the variance in R, while the presence of water in the bark reduced the difference in temperature between the heat source and the vascular cambium, perhaps because of high latent heat of vaporization. This novel finding provides an important insight for improving mechanistic models of fire‐induced cambium damage from tropical to temperate regions. In the second experiment, tree mortality increased with increasing fire intensity (i.e. as indicated by bark char height on tree boles), which was higher along the forest edge, during the 2007 drought, and when the fire return interval was 3 years instead of one. Contrary to other tropical studies, the relationship between mortality and fire intensity was strongest in the year following the fires, but continued for 3 years afterwards. Tree mortality was low (≤20%) for thick‐barked individuals (≥18 mm) subjected to medium‐intensity fires, and significantly decreased as a function of increasing tree diameter, height and wood density. Hence, fire‐induced tree mortality was influenced not only by cambium insulation but also by other traits that reduce the indirect effects of fire. These results can be used to improve assessments of fire vulnerability of tropical forests.