Piet Verburg

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

Mercury biomagnification in the food web of Lake Tanganyika (Tanzania, East Africa)

Lake Tanganyika is a globally important lake with high endemic biodiversity. Millions of people in the lake basin depend on several fish species for consumption. Due to the importance of fish consumption as an exposure route of mercury to humans, we sampled Lake Tanganyika in 2000 to assess total mercury concentrations and biomagnification of total mercury through the food web. Stable nitrogen and carbon isotope analyses of food web structure indicate a complex food web with overlapping omnivory with some specialist fish species. Stable nitrogen isotope analyses further confirm that mercury is biomagnifying through the Tanganyika food web at rates similar to those seen in Lakes Malawi and Victoria, the other two African Great Lakes. Most collected fish species and all invertebrate species had mercury concentrations below 0.2 microg Hg/g wet weight. However, several fish species, Ctenochromis horei (average 0.15 microg/g ww), Neolamprologus boulengeri (0.2 microg/g ww) , Bathybates spp.spp. (0.21 microg/g ww), Mastacembelus cunningtoni (0.22 microg/g ww) and Clarias theodorae (0.22 microg/g ww) approached or slightly exceeded the World Health Organization (WHO)s recommended guideline of 0.2 microg Hg/g for vulnerable populations with high rates of fish consumption. Two individuals of the piscivorous fish species Lates microlepis (0.54, 0.78 microg/g ww) and a Polypterus congicus (1.3 microg/g ww) exceeded the international marketing limit value of 0.5 microg/g ww. Because C. theodorae and L. microlepis are also important market fish species, there is a need to monitor mercury concentrations in internationally marketed fish from Lake Tanganikya to ensure that those fish do not present a risk to human consumers.

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

A stable isotope study of a neotropical stream food web prior to the extirpation of its large amphibian community

Rapid and massive amphibian population declines have been reported throughout upland areas of the Neotropics. The abundance and species richness of Neotropical amphibian communities suggest that losses of this magnitude are likely to have strong effects at the ecosystem level. To improve understanding of the implications of their loss we used stable isotope analysis to examine trophic relationships in an ecosystem in which amphibians are dominant in a second-order forest stream at 750 m asl in Parque Nacional Omar Torrijos Herrera, Panama. We analysed δ 13 C, δ 15 N and C:N ratios of major biotic components (basal resources, invertebrates, amphibians, fish and reptiles) in the stream and of the adjacent riparian food web. Tadpoles (mean δ 15 N = 4.49%o) and adult amphibians (mean δ 15 N = 5.45‰) were intermediate links in the aquatic and terrestrial food web respectively. High δ 15 N signatures identified fish as top predators in the aquatic food web and snakes and the toad Bufo as top predators in the terrestrial food web. Isotopic signatures clearly distinguished between trophic groups of tadpoles: microbial feeders (Centrolenidae, δ 15 N range = 1).91-3.05‰), herbivores (Rana and Hyla, δ 15 N range = 4.74-5.15‰) and neuston feeders (Colostethus, δ 15 N range = 5.31-6.40%o). Dependence on autotrophic production was indicated by enriched signatures of carbon isotopes in pool dwellers versus those that reside in faster-flowing sections of the stream. High nitrogen concentrations in detrital matter (average 0.8%, C:N = 10.3) suggested that grazing tadpoles enhanced nitrogen fluxes and improved the quality of organic matter available to detritivores.

Challenges for interpreting stable isotope fractionation of carbon and nitrogen in tropical aquatic ecosystems

Summary Stable isotopes are useful for elucidating food webs, and oneessentialaspectofinterpretationisaccuratelydeterminingtheenrichment between trophic levels, especially when used inmixing models. The fractionation of the stable isotopes ofnitrogen(Δδ 15 N)andcarbon(Δδ 3 C)betweentrophiclevelsintropicalaquaticecosystemsseemstodiffercomparedtotypicalvalues found in temperate aquatic ecosystems of about 3.4‰forδ 15 Nand0.5‰forδ 13 C.Inrecentstudiesofuplandstreamecosystems in Panama, with and without tadpoles, we foundlower fractionation of δ 15 N, typically 1.0–1.7‰, and muchhigherfractionationofδ 13 C,typically1–1.6‰.Similarvalueshavebeenobservedinothertropicalsystems,includingPuertoRicoandCostaRicastreamsandLakeTanganyika.Theselargedifferencesintrophicfractionationareenigmatic.Weexploresources of variation in fractionation such as tissue turnoverrate,streamflow,taxonomicdifferences,functionalgroupdif-ferences, dietary balance, growth rate, and microbial activityto assess possible contributing factors. The ecological infor-mation embedded in this variation in trophic fractionationshould be embraced and exploited.

Mercury biomagnification in three geothermally-influenced lakes differing in chemistry and algal biomass.

Accumulation of Hg in aquatic organisms is influenced not only by the contaminant load but also by various environmental variables. We compared biomagnification of Hg in aquatic organisms, i.e., the rate at which Hg accumulates with increasing trophic position, in three lakes differing in trophic state. Total Hg (THg) concentrations in food webs were compared in an oligotrophic, a mesotrophic and a eutrophic lake with naturally elevated levels of Hg associated with geothermal water inputs. We explored relationships of physico-chemistry attributes of lakes with Hg concentrations in fish and biomagnification in the food web. Trophic positions of biota and food chain length were distinguished by stable isotope (15)N. As expected, THg in phytoplankton decreased with increasing eutrophication, suggesting the effect of biomass dilution. In contrast, THg biomagnification and THg concentrations in trout were controlled by environmental physico-chemistry and were highest in the eutrophic lake. In the more eutrophic lake frequent anoxia occurred, resulting in favorable conditions for Hg transfer into and up the food chain. The average concentration of THg in the top predator (rainbow trout) exceeded the maximum recommended level for consumption by up to 440%. While there were differences between lakes in food chain length between plankton and trout, THg concentration in trout did not increase with food chain length, suggesting other factors were more important. Differences between the lakes in biomagnification and THg concentration in trout correlated as expected from previous studies with eight physicochemical variables, resulting in enhanced biomagnification of THg in the eutrophic lake.

Potential use of classical biomanipulation to improve water quality in New Zealand lakes: a re-evaluation

The potential use of fish for biomanipulation to improve water quality of lakes in New Zealand was evaluated in 1998 when various constraints, including those based on the structure of pelagic food webs in New Zealand (few potential grazers, zooplanktivores and piscivores), were identified. Since 1998 increased nutrient loading, deteriorating water quality in more lakes and the wider distribution of non-indigenous fish and Daphnia have occurred throughout New Zealand. In light of these changes, we reassess the potential use of biomanipulation to improve water quality and conclude that, whereas constraints related to implementation and management that were outlined in 1998 still remain, subsequent abiotic and biotic changes, including the spread of non-indigenous species, and the possible use of indigenous mussels have strengthened the potential for the use of biomanipulative techniques in integrated lake-specific programmes to improve water quality in some lakes.

Effects of nutrient loading on the trophic state of Lake Brunner

Lake Brunner, an oligotrophic monomictic lake on the West Coast of the South Island of New Zealand, is under pressure from urban expansion and increased farming activity, which has led to concern for the effects on water quality in the lake. Epilimnetic nitrogen, phosphorus and chlorophyll a concentrations have increased since 1992, and Secchi depth decreased. This suggests an increased algal productivity caused by increased nutrient inputs, further supported by increased hypolimnetic oxygen depletion since 1992. These observations are likely to have resulted from enhancement of pasture drainage and effluent inputs from expanding dairy farms. The Vollenweider model predicted a mean phosphorus concentration in the lake close to that observed, from estimated catchment loading, suggesting that the Vollenweider model adequately estimated the retention of phosphorous. With the Vollenweider model the effects of potential future loading scenarios were explored. Modelling suggested that a 70% increase in phosphorus loading could turn the lake into a mesotrophic state. Trend analysis of total phosphorus suggests that, with present land uses in the catchment(intensivedairyfarming)continuingtodevelopatthesamerateusingthesamelandmanagementpractises,this transition to a mesotrophic state will occur by 2040.

Managing pollutant inputs from pastoral dairy farming to maintain water quality of a lake in a high-rainfall catchment

A study (2004–11) of a dairy catchment stream entering an oligotrophic lake in an area of very high rainfall (~5 m year–1) yielded median concentrations of total nitrogen (TN), total phosphorus (TP), suspended sediment (SS) and Escherichia coli (E. coli) of 0.584, 0.074 and 3.7 g m–3, and 405/100 mL (most probable number method), respectively. Trend analysis indicated significant (P < 0.01) decreases for TN (–0.08 ± 0.02 g m–3 year–1), TP (–0.01 ± 0.005 g m–3 year–1) and SS (–0.45 ± 0.14 g m–3 year–1) and were partly attributable to improved exclusion of cattle from the stream. Water balance calculations indicated that approximately one-half the rainfall left as deep drainage that by-passed catchment outlet flow recorders. Estimates of catchment yields for TN were improved by taking into account groundwater hydrology and concentrations from well samples. Storm-flow monitoring inflows exceeding the 97.5th percentile contributed ~40% of total loads leaving the catchment so that specific yields for SS, TN and TP augmented by groundwater inputs and storm flows were ~960, 45 and 7 kg ha–1 year–1, respectively. These compared well with modelled results for losses from dairy farms in the catchment of 40–60 kg N ha–1 year–1 and 5–6 kg P ha–1 year–1 and indicated that attenuation losses were relatively small.

Nutrient ratios, differential retention, and the effect on nutrient limitation in a deep oligotrophic lake

Nutrient ratios have been related to nutrient limitation of algal growth in lakes. Retention of nutrients in lakes, by sedimentation and by denitrification, reduces the nutrient concentrations in the water column, thereby enhancing nutrient limitation. Differential retention of nitrogen and phosphorus alters their ratios in lakes and thereby contributes to determine whether nitrogen or phosphorus limits algal growth. We examined the relationships between differential nutrient retention, nutrient ratios, and nutrient limitation in Lake Brunner, a deep oligotrophic lake. The observed retention of nitrogen (20%) and phosphorus (47%) agreed with predictions by empirical equations from literature. As a result of differential retention with a much larger proportion of phosphorus retained than that of nitrogen, the nitrogen:phosphorus ratio was higher in the lake (69) than in the inflows (46). While the mean ratio in the inflows suggested no or only moderate phosphorus limitation, the lake appeared to be severely phosphorus limited. Combining empirical equations from literature that predict nitrogen and phosphorus retention suggests that the nitrogen:phosphorus ratio is enhanced by greater retention of phosphorus compared to nitrogen only in deep lakes with relatively short residence times, such as Lake Brunner. In contrast, in most lakes differential retention is expected to result in lower nitrogen:phosphorus ratios.