Jeff J. Hudson

Planktonic nutrient regeneration and cycling efficiency in temperate lakes

Planktonic nutrient regeneration is a fundamental process that maintains most of the primary productivity in marine and freshwater environments. However, there is no robust predictive model to describe the pattern and efficiency of nutrient cycling across aquatic systems. Based on rather weak evidence, the efficiency of nutrient regeneration is believed to decline along a gradient of productivity, so that nutrient-poor environments are assumed to be more efficient at cycling their nutrients than are nutrient-rich environments. Here we measure phosphorus regeneration directly and show that cycling efficiency does not vary with phosphorus concentration. In addition, we confirm that the phosphorus supply for lake plankton comes primarily from within the plankton community, rather than from external loading or from larger organisms such as fish.

Determination of Dynamic Metal Complexes and their Diffusion Coefficients in the Presence of Different Humic Substances by Combining Two Analytical Techniques

The combined use of a competing ligand exchange (CLE) method and a diffusive gradient in thin films (DGT) technique in a quasi-labile system provides a better understanding of dynamic metal (Cu and Ni) complexes in the presence of humic substances of different origins. The CLE and DGT techniques provide total labile (dynamic) metal complexes (Cu and Ni) and their dissociation rate constants in environmental systems. DGT was found to estimate lower concentrations of labile metal complexes than CLE. These discrepancies were caused by diffusion controlled metal flux (towards the binding resin gel) in the diffusive gel of DGT. The interactions of Cu and Ni with humic acids are stronger than their interactions with fulvic acid and natural organic matter. Changes in the lability of Ni and Cu complexes (complexed with humic substances of different origins) with the changing analytical detection window indicate that the complexes of these metals were formed with different binding sites with diverse binding energies in the humic substances. The combination of these two techniques was found to be very useful in determining diffusion coefficients of labile metal-humate complexes in quasi-labile systems. The values of diffusion coefficients of labile Ni and Cu complexes determined in this study are in good agreement with limited results from the literature. This finding is novel and can be very useful in further improving our understanding of the metal-humate interactions in natural environments.

Tolerance of low pH exposure by the eggs of Odonata (dragonflies and damselflies)

The development times and hatching success of the eggs of four species of Odonata (Ischnura verticalis, Lestes congener, Libellula lydia, and Sympetrum vicinum) were unaffected by exposure to soft water at pH 5.1 and 3.5. Tolerance of low pH, soft water conditions by Odonata eggs may in part account for the widespread distribution of Odonata in potentially acid-stressed regions.

A comparison of phosphorus deficiency indicators with steady state phosphate in lakes

A fundamental step in the management of nutrient impacted water bodies is the determination of the type and degree of nutrient limitation. However, nutrient deficiency indicators often provide inconsistent results. Recent advances in the measurement of phosphate concentrations may provide a better means to understand results from P deficiency indicators. With regards to phosphorus, deficiency indicators should predict P-limitation when phosphate concentrations are consistently low. We use this new understanding to examine the relationships between phosphate concentration and P deficiency. Patterns of steady state phosphate (ssPO(4)(3-)) concentrations and P deficiency were evaluated in 109 lakes located across Canada. Lakes encompassed a broad range in TP concentration (1.79-139.7 μg L(-1)). The relationships between ssPO(4)(3-) concentrations and simultaneously measured total P (TP), total dissolved P (TDP) and soluble reactive P (SRP) concentrations, particulate C:P and N:P ratios, alkaline phosphatase activities (APA) and phosphate turnover times (TT) were analyzed. ssPO(4)(3-) was positively correlated with TP and TDP. The ssPO(4)(3-) concentrations were 2-3 orders of magnitude lower than SRP concentrations. These two measures were only weakly correlated, suggesting that SRP is a major overestimate of PO(4)(3-). The ssPO(4)(3-) concentrations were negatively correlated with C:P and N:P ratios, and with APA, consistent with expectations. When only lakes with TT < 15 min were considered, TT was negatively correlated with TP, challenging the idea that nutrients become less limiting in more eutrophic systems. Overall, P deficiency indicators related to ssPO(4)(3-) in the expected manner. However, variability in relationships with APA and particulate stoichiometry emphasize the need for cautious interpretation of P deficiency measurements. We recommend simultaneous use of multiple techniques to confidently assess P deficiency.

Phosphorus sedimentation during stratification in two small lakes

Sedimentation of phosphorus was measured with sediment traps during spring through fall at 3 depths in 2 small lakes that had initially different fish and zooplankton communities, but were subject to a manipulation of their fish communities. P sedimentation (P area1 time -1 ) increased with depth in both lakes and all years, and more than accounted for the decline in total P through the stratified season. Sedimentation measured at the base of the epilimnion was sufficient to remove the epilimnetic total P only every 36 days on average, and sedimentation velocities were very low, about 10 to 15 cm d -1 . P sedimentation did not vary significantly between lakes and among years, although it did demonstrate temporal variability. Sedimentation velocity also varied among depths, and was greater in the epilimnion than in the metalimnion. Sedimentation rates were not related to particulate phosphorus concentrations, or to particular size fractions, while sedimentation velocities were only weakly correlated to particle-size distribution. Mass balance calculations suggest that there was substantial input of P to the epilimnion of these small lakes during stratification. This and the resuspension and mineralization of P from the shallow sediments may account for the differences between these results with those for larger lakes and for limnocorrals, respectively.

Improving in-lake water quality modeling using variable chlorophyll a /algal biomass ratios

Abstract Algal simulations in many water quality models perform poorly because of oversimplifications in the process descriptions of the algae growth mechanisms. In this study, algae simulations were improved by implementing variable chlorophyll a/algal biomass ratios in the CE-QUAL-W2 model, a sophisticated two-dimensional laterally-averaged water quality model. Originally a constant in the model, the chlorophyll a/algal biomass ratio was reprogrammed to vary according to the nutrient and light limiting conditions in the water column. The modified model was tested on Lake Diefenbaker, a prairie reservoir in Saskatchewan, Canada, where, similar to many other lakes in the world, field observations confirm variable spatiotemporal ratios between chlorophyll a and algal biomass. The modified version yielded more accurate simulations compared to the standard version and provides a promising algorithm to improve results for many lakes and reservoirs globally.

Do human activities affect phytoplankton biomass and composition in embayments on Lake Diefenbaker

ABSTRACT Lake Diefenbaker (LD) is an important source of water for southern Saskatchewan. LD is characterized by numerous embayments containing anthropogenic activities (e.g., housing, marinas, cattle watering). Many of these activities are increasing on this important reservoir in association with the rapidly developing economy of Saskatchewan. These activities may reduce water quality directly or indirectly by encouraging the growth of nuisance algae (i.e. cyanobacteria). Here, we examined phytoplankton biomass and composition in eight embayments exposed to anthropogenic activities, four unexposed embayments with no perceived human activities and six main channel sites adjacent to the embayments from June to October (2011 and 2012). Phytoplankton biomass and composition was not significantly different in exposed, unexposed embayments and main channel sites (p > 0.05), with the diatoms and cryptomonads constituting 87%–91% of the total phytoplankton biomass in both years. High flows from the South Saskatchewan River (SSR) in both years may have resulted in the rapid flushing of the embayments and dampened any localized impacts that could have resulted from anthropogenic activities as found in other studies. Hence, future study on LD should be conducted during years with low flow from the SSR when the rate of flushing of embayments will be reduced.