Richard P. Barbiero

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: I. Introduction and Phytoplankton Results

The Great Lakes National Program Office of the U.S. EPA has been conducting biological monitoring of the Laurentian Great Lakes since 1983. This paper presents synoptic survey data of phytoplankton communities from all five lakes. These communities were highly diverse, each lake typi-cally supporting over 100 species during both the spring and summer surveys. Much of that diversity was contributed by diatoms, which dominated the plankton of all lakes except Lake Superior in the spring. Summer communities shifted away from diatoms, toward chrysophytes in the upper lakes and chloro-phytes in the lower lakes. Ordination analyses indicated the close similarity of communities in the upper lakes, in particular Lakes Huron and Michigan, and a diverse range of communities in Lake Erie. Floristically, Lake Ontario was fundamentally different from all other lakes.

Long-term Dreissenid Impacts on Water Clarity in Lake Erie

Since shortly after their introduction, dreissenid mussels have been thought to have improved water clarity in Lake Erie, particularly in the western basin. However, long-term monitoring (1982–2004) has found no evidence of persistent, basin-wide increases in water clarity in either the western or the central basin of Lake Erie since the Dreissena invasion. In fact, spring water clarity in both of those basins has exhibited statistically significant declines in the post-dreissenid period. In contrast, chlorophyll a levels in the western basin have declined by about 50% since the Dreissena invasion during both spring and summer. The discrepancy in the responses of water clarity and chlorophyll a is probably a consequence of both the large sediment loads entering the western basin and resuspension of unassimilated non-algal particulates. In the eastern basin, spring transparency has increased substantially and turbidity has decreased since Dreissena colonization, in spite of the much greater depth of this basin. This is probably due to higher mussel densities and the lack of major sources of turbidity in that basin. Summer turbidity has also decreased markedly in the eastern basin, although thermal stratification during this period would probably preclude direct filtration effects. Instead, we hypothesize that reductions in whiting events due to calcium uptake by dreissenids have contributed to the summer decreases in turbidity seen in the eastern basin.

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: III. Crustacean Zooplankton

Open water zooplankton communities were sampled across all five Laurentian Great Lakes during spring and summer 1998. Spring communities were characterized by relatively low species numbers and densities. Crustacean communities in all lakes except Lake Ontario were dominated by diaptomid copepods in spring. During summer, both abundance and species richness increased, the latter owing largely to the appearance of populations of cladocerans. Crustacean communities in the upper lakes were dominated by diaptomid copepods, cyclopoid copepodites, and Daphnia galeata mendotae (codominant with Holopedium gibberum in Lake Superior), and showed a high degree of spatial homogeneity. Lake Erie supported a notably more species rich community, and also exhibited a high degree of spatial heterogeneity. Lake Ontario differed from the other lakes by its relative lack of calanoid copepods, being dominated instead by cyclopoid copepods, along with Bosmina and Daphnia. There was a clear distinction between community composition in the western and eastern portions of the lake, though the reasons for this are unclear.

The Deep Chlorophyll Maximum in Lake Superior

Summer surveys conducted on Lake Superior from 1996–2001 indicated that a deep chlorophyll maximum (DCM) is a common feature of the offshore waters. The DCM was usually observed in the upper hypolimnion between 23 and 35 m, a region lacking a pronounced density gradient. Chlorophyll a concentrations in the DCM were typically 1.5–2.5 (median = 2.0) times epilimnetic concentrations, although these were associated with minimal or no increases in particulate organic carbon concentrations. Seston carbon:phosphorus ratios were consistently lower in the DCM than in the epilimnion, indicating increased phosphorus content of DCM phytoplankton. This could have resulted from either improved nutrient conditions or light limitation at depth. A phosphorus-rich phytoplankton community at depth could serve as a resource for the large, deep-living calanoid copepods that constitute the majority of summer zooplankton biomass. The phytoplankton communities at the level of the DCM were taxonomically distinguishable from those in the epilimnion, with the most notable difference being a relative reduction in the abundance of Cyclotella species in the DCM.

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: II. Deep Chlorophyll Maxima

Deep chlorophyll maxima (DCM) were found in all five Laurentian Great Lakes during August, 1998. Chlorophyll profiles were consistent over large areas in Lakes Superior and Michigan, while distinct inter-site differences were apparent in the other three lakes. Shade adaptation appeared to be primarily responsible for increases in chlorophyll at depth in Lakes Huron and Ontario, while in Lake Superior increases in phytoplankton biovolume were also noted. Deep living phytoplankton populations in the latter lake exhibited improved nutrient status at depth, where concentrations of both soluble phosphorus and silica were higher. Phytoplankton community composition in the DCM differed from that previously reported for the lakes, most notably in the reduced populations of Cyclotella, relative to the epilimnion, seen at most sites. Filamentous chlorophytes were often more abundant at depth, as were certain species of Dinobryon.

Post-dreissenid Increases in Transparency During Summer Stratification in the Offshore Waters of Lake Ontario: Is a Reduction in Whiting Events the Cause?

ABSTRACT Since the dreissenid invasion of the lower Great Lakes, calcium concentrations in the offshore waters of Lake Ontario have decreased by approximately 4–5 mg/L. This decline has coincided with a three-fold reduction in August turbidity values and nearly a doubling of Secchi depths, presumably due to reduced summer calcite precipitation events in the lake. The reductions in calcium have followed a dramatic reduction in alkalinity in the central and eastern basins of Lake Erie, which provides most of the inflow to Lake Ontario. This reduction in alkalinity in Lake Erie corresponds to a period of rapid dreissenid growth in that lake, strongly suggesting calcium uptake by dreissenid mussels as a causative factor. The mass of calcium resident in the dreissenid population in Lake Erie, estimated from published lake-wide census data, is sufficient to account for the observed decreases in alkalinity. In addition, observed changes in alkalinity in Lake Ontario closely match those expected to result from inflows from Lake Erie, based on mass balance considerations. Considered in sum, our data strongly suggest that calcium uptake by dreissenid mussels in Lake Erie has resulted in decreases in the calcium concentration in Lake Ontario, reducing the frequency and/or intensity of whiting events in the latter lake. We believe this is the first report of an increase in transparency that can be reasonably attributed to a chemical change brought about by Dreissena. These increases in transparency may have very different consequences than those of dreissenid filtration activities. For example, rather than decreasing phytoplankton populations, the improved light climate might increase summer phytoplankton populations, particularly sub-epilimnetic ones.

Recent increases in the large glacial-relict calanoid Limnocalanus macrurus in Lake Michigan

ABSTRACT Since 2004, population density of the large hypolimnetic calanoid Limnocalanus macrurus Sars. has increased dramatically in Lake Michigan. The average summer biomass of this species between 2004 and 2006 was roughly three times that of the period 1984–2003, and at levels unprecedented in our 22-year dataset, making L. macrurus the dominant zooplankter in the lake in terms of biomass. These increases have been accentuated by coincident population declines of the main daphnid, Daphnia mendotae, in the lake with the result that in 2006, L. macrurus accounted for 75% and 50% of the large (>0.9 mm) crustacean biomass in the northern and southern basins of Lake Michigan, respectively. The increases in L. macrurus populations have closely coincided with equally dramatic increases in summer water clarity. Recent extinction coefficients are among the lowest recorded for the lake, and deepening light penetration has permitted increases in the size of the deep chlorophyll layer. In addition, planktivorous fish populations have declined coincidently with the increases in L. macrurus. It seems likely that an increase in sub-epilimnetic production has resulted in increased food resources for the deep-living L. macrurus, while low planktivore abundances have reduced predation loss, permitting L. macrurus to respond to these increases in sub-epilimnetic production.

Distribution and Population Characteristics of Cercopagis pengoi in Lake Ontario

The spatial and vertical distribution of a recent exotic species, the predatory cladoceran Cercopagis pengoi, was studied in Lake Ontario in September 1999. Only typical forms of the species C. pengoi, characterized by a relatively long tail with an S-bend and claws with straight or backwardly bent tips, were found. Structure of the Cercopagis population was rather uniform over the lake, consisting mainly (over 90%) of parthenogenetic females. Median epilimnetic abundance and biomass was 295/m3 (max. = 2,544/m3) and 13.4 mg DW/m3 (max. = 113.3 mg DW/m3), respectively. Cercopagis contributed a median of 15.8%, and at maximum 73.8%, of the total crustacean zooplanktonic biomass (exclusive of nauplii). Abundances showed a significant positive relationship with distance from shore (r2 = 0.34, p < 0.01), but distribution was independent of the depth and temperature of the epilimnion. Cercopagis did not exhibit any diurnal vertical migration patterns: over 90% of the individuals stayed either in the epilimnion or within the metalimnion during the day and night. The proportion of individuals, both live specimens and carcasses of dead individuals, in cooler layers was negligible (< 3%). The following weight (W) – body length (L) relationship was developed during the study: ln(W) = 2.98*ln(L) - 6.42 (r2 = 0.85, p < 0.001).

Variation in Lake Michigan Plankton: Temporal, Spatial, and Historical Trends☆

Abstract Lake Michigan has been impacted by excessive material loading and invasion by exotic species; however, few studies have evaluated the recent basin-wide response of the lake to these changes, particularly given the reduction of phosphorus loads since the 1970s. From 1994–95, quarterly measurements were made of physical-chemical conditions, plankton biomass, and plankton species composition at 18 stations throughout the lake (n = 111). Sampling sites were clustered according to their physical-chemical similarity; these zones corresponded with depositional regions in the lake (Deep water, Shallow water, and Impacted regions). While plankton biomass did not vary among lake-zones, species composition was zone-specific suggesting that several factors (food web structure, nutrient cycling, and physical mixing) may determine the distribution of species throughout the lake. Plankton biomass and gross composition (phyla) were variable in time (seasons), and exhibited predictable succession patterns. Phytoplankton peaked in June corresponding with the upward mixing of nutrients, while zooplankton peaked during mid-stratification (August) when water temperatures were most warm. Finally, the basin-wide estimates for both total phosphorus and phytoplankton biomass were lower compared with historical estimates (measured in 1970s) and significant differences were not observed between near- and offshore regions. Despite this, the data also show that phytoplankton species composition varies widely throughout the lake, and that some nearshore sites do support impacted assemblages.

Ecological monitoring for assessing the state of the nearshore and open waters of the Great Lakes

The Great Lakes Water Quality Agreement stipulates that the Governments of Canada and the United States are responsible for restoring and maintaining the chemical, physical and biological integrity of the waters of the Great Lakes Basin Ecosystem. Due to varying mandates and areas of expertise, monitoring to assess progress towards this objective is conducted by a multitude of Canadian and U.S. federal and provincial/state agencies, in cooperation with academia and regional authorities. This paper highlights selected long-term monitoring programs and discusses a number of documented ecological changes that indicate the present state of the open and nearshore waters of the Great Lakes.