Kenton M. Stewart

Extreme events, trends, and variability in Northern Hemisphere lake-ice phenology (1855-2005)

Often extreme events, more than changes in mean conditions, have the greatest impact on the environment and human well-being. Here we examine changes in the occurrence of extremes in the timing of the annual formation and disappearance of lake ice in the Northern Hemisphere. Both changes in the mean condition and in variability around the mean condition can alter the probability of extreme events. Using long-term ice phenology data covering two periods 1855–6 to 2004–5 and 1905–6 to 2004–5 for a total of 75 lakes, we examined patterns in long-term trends and variability in the context of understanding the occurrence of extreme events. We also examined patterns in trends for a 30-year subset (1975–6 to 2004–5) of the 100-year data set. Trends for ice variables in the recent 30-year period were steeper than those in the 100- and 150-year periods, and trends in the 150-year period were steeper than in the 100-year period. Ranges of rates of change (days per decade) among time periods based on linear regression were 0.3−1.6 later for freeze, 0.5−1.9 earlier for breakup, and 0.7−4.3 shorter for duration. Mostly, standard deviation did not change, or it decreased in the 150-year and 100-year periods. During the recent 50-year period, standard deviation calculated in 10-year windows increased for all ice measures. For the 150-year and 100-year periods changes in the mean ice dates rather than changes in variability most strongly influenced the significant increases in the frequency of extreme lake ice events associated with warmer conditions and decreases in the frequency of extreme events associated with cooler conditions.

Chironomid Deformities, Benthic Community Composition, and Trace Elements in the Buffalo River (New York) Area of Concern

Deterioration of ecosystem health is often reflected by changes in benthic community structure, but specific community metrics may differ widely in their usefulness as environmental monitors. We investigated the association of several of these metrics with concurrently measured sediment trace element levels at 15 sites within the Buffalo River, New York from 1990 to 1993. Site mean concentrations (9 sample dates) of As, Cd, Cu, Fe, Hg, Mn, Ni, Pb, and Zn were subjected to principal component analysis, yielding 1 factor describing a spatial gradient in trace element levels. Community metrics were then regressed on this factor. The mean density of the family Chironomidae decreased (R2 = 0.41, p = 0.01) with increasing factor scores (i.e., higher trace element concentrations). However, the mean density of tubificid oligochaetes, which accounted for >80% of the benthic community, was not linearly associated with trace elements (R2 = 0.22, p = 0.08). Mean richness and diversity of the chironomid community were strongly negatively associated with factor scores (R2 = 0.77 and 0.76, respectively, p < 0.001), while the prevalence of the tolerant genera Procladius and Chironomus increased with trace element levels (R2 = 0.55, p = 0.002). Mouthpart deformities in larvae of Chironomus thummi group also increased with higher trace element levels (R2 = 0.72, p < 0.001). Benthic community metrics can be very useful in ranking the health of specific sites, but our study shows that more detailed metrics (e.g., taxonomically detailed chironomid data, Chironomus mouthpart deformity frequencies) provide additional information on community health that justifies the extra effort required for their assessment.

Deformities of Aquatic Larval Midges (Chironomidae: Diptera) in the Sediments of the Buffalo River, New York

We conducted an extensive survey of the benthic macroinvertebrate fauna of the heavily polluted Buffalo River (New York) Area of Concern (AOC), with specific emphasis on assessment of morphological deformities of aquatic larval midges (Chironomidae: Diptera). Sediments were sampled on seven occasions during 1990 - 1991, from as many as 20 sites. Abnormal mouthparts (menta) were most frequent among larvae of the genus Chironomus, occurring in 29.0% of all specimens in the Buffalo River (n = 755), and 22.4% in the adjacent Ship Canal (n = 98). Individual sites in the downstream section of the river yielded even higher deformity frequencies; 36.9% (n = 84), 48.1% (n = 27), and 66.7% (n = 9). The frequencies of abnormalities in this genus at all sites within the AOC were well above levels normally observed (0 - 3%) at non-industrial sites. Additionally, abnormal mouthparts (ligulae) were seen in 3.1% of larvae of the genus Procladius (n = 1,677). While individual downstream samples of Procladius were occasionally as much as 15.4% abnormal (n = 26), this genus was much less prone to morphological deformities than Chironomus. Abnormal menta were also noted in the genera Polypedilum, Cryptochironomus, and Dicrotendipes, but were infrequent. Community parameters such as richness and diversity of the chironomid fauna, and the prevalence of oligochaetes, provide further evidence of decreased environmental quality within the downstream reaches of the Buffalo River. The data presented here provide one biologically relevant measure against which to gauge the effectiveness of pollution abatement strategies within the AOC.

Epiphytic Refugium: Are Two Species of Invading Freshwater Bivalves Partitioning Spatial Resources?

Enumeration of benthic (bottom dwelling) and epiphytic (attached to plants) zebra and quagga mussels (Dreissena polymorpha and D. bugensis, respectively) at Lake Erie near-shore sites in fall of 2000 revealed an unexpected prevalence of the zebra mussel on submerged plants. Even at Buffalo, New York, USA, where benthic dreissenids have been 92–100% quagga mussel since 1996, zebra mussels constituted 30–61% of epiphytes numerically. This may reflect a partitioning of settling space consistent with interspecific competition. A seasonal epiphytic refugium might allow the zebra mussel to persist even where the benthos is almost exclusively quagga mussel.

Persistence of coherence of ice-off dates for inland lakes across the Laurentian Great Lakes region

lee phenologies, dates of ice-on and ice-off on lakes and rivers, provide information on climate change and variability. These long-term records indicate that lakes and rivers are sensitive to climatic change and variability. A symposium on ice phenologies o f lakes as a climate indicator was held at the 271h Congress of the Intemational Association of Theoretical and Applied Limnology in Dublin and published in the Verhandlungen (MAGNUSON et al. 2000b ). lee records have been especially useful because they can be long, over 150 years in length, and occur broadly around the Northem Hemisphere. In some areas, such as Finland (Kuustsro & ELO 2000), Sweden (WEYHENMEYER et al. 2004), and the Great Lakes region of North America (MAGNUSON et al. 2005a, b), lakes with records o f moderate length are common enough so that spatial pattems in dynamics can be analyzed. In some cases, records are sufficient to examine global, or at least intercontinental, trends (MAGNUSON et al. 2000a) and dynamics (LIVINGSTONE 2000, MAGNUSON et al. 2004). Our purpose here is to examine the spatial pattems o f temporal coherence or synchrony in time series o f ice-off dates between lakes in the Laurentian Great Lakes region. In particular, we (l) compare the coherence within and between four states and one province bordering the Great Lakes, (2) describe the persistence and decline of coherence between lakes with increasing latitudinal and longitudinal distances between them, and (3) suggest explanations for the observed pattem of coherence at multiple spatial scales. Coherence, as quantified here, is the shared variance between two time series. Discussions of coherent dynamics between lakes are reviewed in MAGNUSON & KRATZ (2000) and MAGNUSON et al. (2005b).