The impact of calcium-rich diamond mining effluent on downstream cladoceran communities in softwater lakes of the Northwest Territories, Canada

Effluent from diamond mining operations rich in calcium (Ca) has transformed softwater tundra lakes in the Northwest Territories, Canada. Lakes downstream of the Dominion Diamond Corporation Ekati Mine have experienced marked changes in water chemistry and cladoceran community composition since establishment of the mine in 1998. The greatest changes have occurred at the sites closest to the effluent discharge, with [Ca] increasing from <1 to >30 mg·L−1 and corresponding increases in pH from <7 to >8. A split was identified in the cladoceran communities with Holopedium glacialis (a jelly-clad cladoceran tolerant of low [Ca]) generally dominating the cladoceran community at [Ca] < 2.5 mg·L−1, while in impacted lakes with [Ca] ≥ 2.5 mg·L−1, Daphnia longiremis and Daphnia middendorffiana (taxa with higher [Ca] requirements) were often dominant. In contrast, the three study lakes that did not receive mining effluent maintained stable and low [Ca] (mean [Ca] = 0.66 ± 0.06 mg·L−1 (SD)) throughout the monitoring period and have not experienced directional shifts in their cladoceran communities.

The relationships between structural and functional diversity within and among macrophyte communities in lakes

We studied the relationships between structural and functional diversity within and among macrophyte communities of softwater lakes. Diversity was assessed based on Simpson’s diversity index (SD), species richness (S), Rao’s functional diversity (FDQ) and shared plant traits along a gradient of pH. A total of 10,800 cover-plant samples (area=0.1 m2) were analysed, collected from 241 sections of the bottom (with a depth resolution of 1.0 m) of 38 lakes in Poland. We identified 59 species of macrophytes and recognized four communities. Along a gradient of increasing pH we found i) an increase of species richness; ii) a uniformity of abundance in communities; and iii) an increase in the quantitative contribution of life history traits such as perennials, submerged anchoring, leaf placement along an orthotropic shoot and generative reproduction. On the other hand, there was a decrease in unanchored and evergreen perennials. In our study, we found that the variety of these traits within communities is greater than that between them. Within lakes, structural diversity was low, whereas it was high between lakes. Functional diversity exhibited an opposite pattern: it was high at lake scale, and low between lakes. A combination of partitioning diversity and utilizing environmental variables significantly improves the prediction of community structure and the conservation of lakes.

The jellification of north temperate lakes

Calcium (Ca) concentrations are decreasing in softwater lakes across eastern North America and western Europe. Using long-term contemporary and palaeo-environmental field data, we show that this is precipitating a dramatic change in Canadian lakes: the replacement of previously dominant pelagic herbivores (Ca-rich Daphnia species) by Holopedium glacialis , a jelly-clad, Ca-poor competitor. In some lakes, this transformation is being facilitated by increases in macro-invertebrate predation, both from native ( Chaoborus spp . ) and introduced ( Bythotrephes longimanus ) zooplanktivores, to which Holopedium , with its jelly coat, is relatively invulnerable. Greater representation by Holopedium within cladoceran zooplankton communities will reduce nutrient transfer through food webs, given their lower phosphorus content relative to daphniids, and greater absolute abundances may pose long-term problems to water users. The dominance of jelly-clad zooplankton will likely persist while lakewater Ca levels remain low.

Impact of humic substances on Sphagnum denticulatum habitats

In 10 softwater lakes of NW Poland (5 very acid and 5 slightly acid) three kinds of <em>Sphagnum denticulatum</em> Bridel [=<em>S. auriculatum</em> Schimper] habitats have been studied: oligohumic (&lt;2.0 mg C_DHS dm^-3) and α-mesohumic (2.0-4.0 mg C_DHS dm^-3) and β-mesohumic (4.1-8.0 mg C_DHS dm^-3). During habitats humification in very acid lakes some water and sediment characteristics, such as pH, conductivity, TDS, and calcium concentration are low and almost invariable, whereas variable are the colour and clearness of the water. In low-acidity lakes the values of habitat characteristics are higher and also nearly invariable. The optimum habitats for <em>S. denticulatum</em> are oligohumic (&lt;1.8 mg C_DHS dm ^-3), very acid (pH 4.0-4.4) sites found at depths from 4 to 8 m, where the species forms large and close carpets. In slightly acid lakes it is rare, grows at depths below 5 m and forms populations of very low density. In mesohumic habitats in both kinds of lakes <em>S. denticulatum</em> is a frequent species, but its populations are very thin there.

Changes in water chemistry can disable plankton prey defenses

The effectiveness of antipredator defenses is greatly influenced by the environment in which an organism lives. In aquatic ecosystems, the chemical composition of the water itself may play an important role in the outcome of predator–prey interactions by altering the ability of prey to detect predators or to implement defensive responses once the predator’s presence is perceived. Here, we demonstrate that low calcium concentrations (<1.5 mg/L) that are found in many softwater lakes and ponds disable the ability of the water flea, Daphnia pulex to respond effectively to its predator, larvae of the phantom midge, Chaoborus americanus. This low-calcium environment prevents development of the prey’s normal array of induced defenses, which include an increase in body size, formation of neck spines, and strengthening of the carapace. We estimate that this inability to access these otherwise effective defenses results in a 50–186% increase in the vulnerability of the smaller juvenile instars of Daphnia, the stages most susceptible to Chaoborus predation. Such a change likely contributes to the observed lack of success of daphniids in most low-calcium freshwater environments, and will speed the loss of these important zooplankton in lakes where calcium levels are in decline.