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We studied the potential role of dissolved inorganic carbon (DIC) in determining vegetation dominance of Potamogeton pectinatus L. and Chara aspera Deth. ex Willd. by monitoring the seasonal dynamics of DIC in a shallow lake and comparing the use of DIC of the two species. The HCO3−-concentration in summer dropped from 2.5 to <0.5 mM with seasonally increasing Chara biomass, whereas outside the vegetation concentrations remained at 2.5 mM. Inside Potamogeton spp. vegetation DIC decreased from 2.5 to ca. 0.75 mM HCO3−. A growth experiment showed ash-free biomass for P. pectinatus was nearly two times as high as for C. aspera at 3 mM HCO3−, but almost two times lower at 0.5 mM than at 3.0. In a separate experiment, P. pectinatus precultured at a relatively low HCO3−-level had a lower net photosynthetic rate (Pmax, 0.1 mmol O2 g−1 DW h−1) than C. aspera (Pmax, 0.1 mmol O2 g−1 DW h−1) over the range of HCO3−-concentrations tested (Pmax, 0.14 mmol O2 g−1 DW h−1). In response to CO2 no significant differences between the compensation points (P. pectinatus, 28 mM; C. aspera 66 mM), were observed, but the photosynthetic rate increased faster than for C. aspera than for P. pectinatus. Under field conditions, the use of CO2 is not important since inside vegetation CO2-concentrations were below 10 μM, and thus, not available for photosynthesis of either species during the main part of the growth season. It is suggested that C. aspera may be a better competitor for HCO3− than P. pectinatus in conditions with a low HCO3− supply. As HCO3− is a strong limiting factor for growth inside the vegetation and probably the only carbon source available, the superior ability of C. aspera to use HCO3− may be an important factor explaining its present dominance in Veluwemeer. [Copyright &y& Elsevier]

An important problem in lake management is the replacement of submerged plant species by competing species with undesirable properties. Although dynamics of rooted aquatic macrophytes have been extensively modeled, competition between different species has hardly been addressed so far. We present an individual-based spatially explicit model (‘Charisma’) designed to simulate the growth of one or more competing species of submerged macrophytes. The model behavior easily becomes rather complex and puzzling if we study more than one species. This probably reflects the complexity as it really occurs in nature, but poses a challenge for calibrating and understanding the model. To control this complexity, we used a flexible design facilitating the elimination of feedback loops and processes to trace the causes of the complex behavior of the model. The resulting ‘realistic, yet transparent’ model appeared useful in many aspects for bridging the existing gap between theory and reality. As an example, we present an analysis of competition between the charophyte Chara aspera Deth. ex Willd. and pondweed Potamogeton pectinatus L. The model suggests that alternative equilibria may arise as a result of two positive feedbacks: the enhancement of transparency by macrophytes and a feedback caused by bicarbonate competition. [Copyright &y& Elsevier]

Water-birds were almost absent from Lake Veluwemeer during the 1970s and 1980s, due to low food availability related to eutrophication. Several restoration measures have resulted in the recolonization of the lake by Chara spp. during the 1990s. Bird numbers in autumn and winter have increased along with charophyte biomass. Linear regression analysis was used to relate bird numbers to the abundance of their potential food sources Chara, pondweeds, filamentous macro-algae (FA) and zebra mussels (Dreissena polymorpha) within the depth range available to each species. Numbers of mute swan (Cygnus olor), pochard (Aythya ferina), tufted duck (A. fuligula), coot (Fulica atra) (R2≥0.98), gadwall (Anas strepera) (R2=0.78), Bewick’s swan (Cygnus columbianus) (R2=0.79), red-crested pochard (Netta rufina) (R2=0.76) and pintail (Anas acuta) (R2=0.38) showed significant correlations with these food sources. Chara biomass explained most of the variance in all species except for gadwall. Both bird numbers and their duration of stay were closely associated with the presence of Chara. Grazing pressure was low during spring and summer and Chara colonized the lake in spite of consumption. It is argued that birds did not slow down colonization of the lake by Chara during the 1990s, but may have accelerated succession from pondweeds Potamogeton spp. to charophytes. As more birds foraged on Chara compared to any other food resource, the preservation of large areas of Chara is of great importance to the water-bird community in Lake Veluwemeer. [Copyright &y& Elsevier]

Promoting indigenous submerged macrophytes is considered an important measure to restore shallow lakes. On the other hand, dense stands of aquatic vegetation often cause nuisance for boating, swimming and by obstruction of water flow. Consequently, the interests of recreational users may conflict with nature conservation. At first sight, aiming lake management at intermediate vegetation biomass seems a good solution for this controversy. However, as can be shown with a simple economical model, such a compromise may not always be an optimal solution in terms of total welfare across all users of a lake. Here, we show that a management strategy aimed at moderate plant biomass can also be unfeasible for ecological reasons. This is shown by applying two very different models: a minimal logistic model of plant growth which is easy to understand, and the relatively complex and realistic simulation model Charisma which, among other things, includes the seasonal cycle and a detailed description of the dynamics of light availability for macrophytes. The qualitative results of both models were remarkably similar. If the ecosystem has no alternative stable states, it is possible to set vegetation to any desired sustainable level using an appropriate harvesting strategy. However, if an intermediate vegetation biomass is to be realized the costs of harvesting are high, because the yield is predicted to be maximal in that case. If the ecosystem has alternative stable states, harvesting becomes risky because the vegetation may collapse entirely below a certain, in practice unknown, biomass. Also, even moderate harvesting may reduce the resilience of the vegetated state, making the ecosystem more vulnerable to adverse conditions such as unfavorable weather conditions or bird grazing. [Copyright &y& Elsevier]

Submerged plant dominance and a turbid state with few submerged plants have been hypothesized to represent alternative stable states in eutrophic shallow lakes. Here, we analyze the conditions for occurrence of alternative stable states in shallow lakes further, using Charisma, a simulation model describing the growth of Chara aspera. The model includes seasonality and spatial structure, aspects which were absent in earlier models predicting alternative equilibria. The parameterization of the model is largely based on experimental results and field observations. Over a range of conditions, the model does indeed predict alternative stable states. The range of conditions over which alternative stable states exist, appeared most sensitive to the assumed reduction of local turbidity by plants and the maximum growth rate of the plants. Aboveground biomass disappears during winter in most lakes in temperate regions. Our analysis indicates that from an evolutionary perspective there is an optimum biomass allocation strategy with respect to investment in overwintering structures. Too little investment reduces chances to regain dominance in the subsequent year, whereas too much investment in dormant overwintering structures such as seeds and tubers reduces photosynthesis. Interestingly, the optimal investment is lower for obtaining maximal summer biomass than for realizing the maximum stability of the vegetated state.The model also suggests that a short clear-water phase enhances the probability of vegetation survival. The optimal timing for a clear-water phase is at the end of May or in June, as is indeed the case in many lakes.In line with earlier theory, shallow lakes with a ‘flat’ depth profile are predicted to have the strongest hysteresis. In lakes with a depth gradient, the response to changes in turbidity is predicted to depend strongly on horizontal mixing of the water between vegetation stands and the open water. Hysteresis disappears predominantly due to a strong horizontal mixing of water. In case of little mixing, on the other hand, local alternative stable states are predicted to occur. [Copyright &y& Elsevier]

Abstract Passive acoustic monitoring was used to detect the sounds of rarely sighted Antarctic blue and fin whales to investigate their seasonal occurrence (as presence or absence of whale calls) and behaviour (as determined from call rates) in the Benguela ecosystem. Data were collected using autonomous acoustic recorders deployed on oceanographic moorings for 16.26 months off the west coast of South Africa in 2014 and 2015. Satellite derived environmental variables were used as predictors of whale acoustic occurrence and behaviour. Migratory Antarctic blue and fin whales were acoustically present in South African waters between May and August with call occurrence peaks in July whereas some fin whales extended their presence to November. No whale calls were recorded in summer for either species, suggesting whales use the Benguela ecosystem as an overwintering ground and migration route. Antarctic blue whales produced both their characteristic Z -call and their feeding associated D-call. Fin whales produced calls characteristic of animals from the eastern Antarctic fin whale acoustic population. Random forest models identified environmental variables such as sea surface temperature anomaly, sea surface height, wind speed, months of the year, Ekman upwelling index and log-transformed chlorophyll-a as the most important predictors of call occurrence and call rates of blue and fin whales. Here we present the first acoustic recordings of Antarctic blue and fin whales in the Benguela ecosystem, and provide preliminary information to investigate seasonal abundance and distribution of these large baleen whale populations. This work demonstrates the feasibility of cost-effectively monitoring Antarctic top-consumer baleen whales in the Benguela ecosystem. Highlights • Whale calls were detected from May to November but peaked in July. • First acoustic records of sympatric whale occurrence off the west coast of South Africa. • Acoustic evidence of Antarctic blue whales feeding in the Benguela ecosystem • Call occurrences indicate the west coast of South Africa as an overwintering ground and migration route. • Environmental conditions influenced whale call occurrence and diel acoustic behaviour. [ABSTRACT FROM AUTHOR]

The European Water Framework Directive has been adopted by Member States to assess and manage the ecological integrity of surface waters. Specific challenges include harmonizing diverse assessment systems across Europe, linking ecological assessment to restoration measures and reaching a common view on ‘good’ ecological status. In this study, nine national macrophyte-based approaches for assessing ecological status were compared and harmonized, using a large dataset of 539 European lakes. A macrophyte common metric, representing the average standardized view of each lake by all countries, was used to compare national methods. This was also shown to reflect the total phosphorus (r 2  = 0.32), total nitrogen (r 2  = 0.22) as well as chlorophyll- a (r 2  = 0.35–0.38) gradients, providing a link between ecological data, stressors and management decisions. Despite differing assessment approaches and initial differences in classification, a consensus was reached on how type-specific macrophyte assemblages change across the ecological status gradient and where ecological status boundaries should lie. A marked decline in submerged vegetation, especially Charophyta (characterizing ‘good’ status), and an increase in abundance of free-floating plants (characterizing ‘less than good’ status) were the most significant changes along the ecological status gradient. Macrophyte communities of ‘good’ status lakes were diverse with many charophytes and several Potamogeton species. A large number of taxa occurred across the entire gradient, but only a minority dominated at ‘less than good’ status, including filamentous algae, lemnids, nymphaeids, and several elodeids (e.g., Zannichellia palustris and Elodea nuttallii ). Our findings establish a ‘guiding image’ of the macrophyte community at ‘good’ ecological status in hard-water lakes of the Central-Baltic region of Europe. [ABSTRACT FROM AUTHOR]

The Water Framework Directive is the first international legislation to require European countries to establish comparable ecological assessment schemes for their freshwaters. A key element in harmonising quality classification within and between Europe's river basins is an “Intercalibration” exercise, stipulated by the WFD, to ensure that the good status boundaries in all of the biological assessment methods correspond to similar levels of anthropogenic pressure. In this article, we provide a comprehensive overview of this international comparison, focusing on the assessment schemes developed for freshwater lakes. Out of 82 lake ecological assessment methods reported for the comparison, 62 were successfully intercalibrated and included in the EC Decision on intercalibration, with a high proportion of phytoplankton (18), macrophyte (17) and benthic fauna (13) assessment methods. All the lake assessment methods are reviewed in this article, including the results of intercalibration. Furthermore, the current gaps and way forward to reach consistent management objectives for European lakes are discussed. [ABSTRACT FROM AUTHOR]

The fish community in the shallow Lake Veluwe (3400 ha) has recently changed in composition. The total biomass of fish decreased to ca. 35% of the original biomass. The change in fish biomass followed the introduction of a commercial fishery and was in its turn followed by a rapid expansion of macrophyte beds dominated by Chara aspera Deth. ex Willd.An individual-based fish model was used to evaluate the causation of these events. We estimated natural mortality of the bream population (Abramis brama L.) using data on growth, recruitment and size distribution. The model predicted that without the fishery the biomass of the bream population would hardly change and that the fishery could explain the change in the biomass and size composition.The rapid expansion of C. aspera was unlikely without the fishery, as Chara needs transparency down to the bottom and undisturbed sediment. The reduction of the bream population created favourable conditions for zebra mussels Dreissena polymorpha (Pallas) to settle which in turn created the clear water, necessary for the development of the Chara beds. [Copyright &y& Elsevier]