Kristen T. Holeck

Bridging Troubled Waters: Biological Invasions, Transoceanic Shipping, and the Laurentian Great Lakes

Abstract Release of contaminated ballast water by transoceanic ships has been implicated in more than 70% of faunal nonindigenous species (NIS) introductions to the Great Lakes since the opening of the St. Lawrence Seaway in 1959. Contrary to expectation, the apparent invasion rate increased after the initiation of voluntary guidelines in 1989 and mandatory regulations in 1993 for open-ocean ballast water exchange by ships declaring ballast on board (BOB). However, more than 90% of vessels that entered during the 1990s declared no ballast on board (NOBOB) and were not required to exchange ballast, although their tanks contained residual sediments and water that would be discharged in the Great Lakes. Lake Superior receives a disproportionate number of discharges by both BOB and NOBOB ships, yet it has sustained surprisingly few initial invasions. Conversely, the waters connecting lakes Huron and Erie are an invasion hotspot despite receiving disproportionately few ballast discharges. Other vectors, including canals and accidental release, have contributed NIS to the Great Lakes and may increase in relative importance in the future. Based on our knowledge of NIS previously established in the basin, we have developed a vector assignment protocol to systematically ascertain vectors by which invaders enter the Great Lakes.

Spatial and long-term temporal assessment of Lake Ontario water clarity, nutrients, chlorophyll a, and zooplankton

Secchi depth, total phosphorus, soluble reactive phosphorus, silica, chlorophyll a, and zooplankton (density, biomass, and average size) were sampled as part of a lake-wide, seasonal (spring, summer, and fall) assessment of Lake Ontario in 2003 to characterize the status of the lower aquatic food web. For each parameter, spatial comparisons were performed to test for differences between habitats (nearshore and offshore) and between regions (east and west) during each season. Significant differences between habitats were found only for silica and chlorophyll a; silica was higher nearshore in fall, and chlorophyll a was higher offshore in fall. Significant differences between regions were detected in Secchi depth, epilimnetic zooplankton mean length, total phosphorus and Cercopagis pengoi density and biomass; Secchi depth and zooplankton mean length were higher in the east in spring, total phosphorus and Cercopagis pengoi biomass were higher in the west in summer, and Cercopagis pengoi biomass was higher in the east in fall. Cercopagis pengoi was present lake-wide in summer and fall, but Bythotrephes longimanus was present only in fall in the Kingston basin. Mean spring total phosphorus, soluble reactive phosphorus, chlorophyll a, and epilimnetic zooplankton density and biomass are at or near record low levels. As we move into the future, persistent low levels of these lower food web elements will continue to stress alewife populations both through reduced food resources and food quality for zooplankton, and may force these fish to seek alternative food such as Mysis.

The Lake Ontario Zooplankton Community before (1987–1991) and after (2001–2005) Invasion-Induced Ecosystem Change

ABSTRACT We assessed changes in Lake Ontario Zooplankton biomass, production, and community composition before (1987–1991) and after (2001–2005) invasion-induced ecosystem changes. The ecosystem changes were associated with establishment of invasive dreissenid mussels and invasive predatory cladocerans (Bythotrephes and Cercopagis). Whole-lake total epilimnetic plus metalimnetic Zooplankton production declined by approximately half from 42.45 (g dry wt·m-2·year-1) during 1987–1991 to 21.91 (g dry wt·m-2·year-1) in 2003 and averaged 21.01 (g dry wt·m-2·year-1) during 2001–2005. Analysis of two independent data sets indicates that the mean biomass and biomass proportion of cyclopoid copepods declined while the same measures increased for the invasive predatory cladocerans. Changes in means and proportions of all other Zooplankton groups were not consistent between the data sets. Cyclopoid copepod biomass and production declined by factors ranging from 3.6 to 5.7. Invasive predatory cladoceran biomass averaged from 5.0% to 8.0% of the total Zooplankton biomass. The Zooplankton community was otherwise resilient to the invasion-induced disruption as Zooplankton species richness and diversity were unaffected. Zooplankton production was likely reduced by declines in primary productivity but may have declined further due to increased predation by alewives and invasive predatory cladocerans. Shifts in Zooplankton community structure were consistent with increased predation pressure on cyclopoid copepods by alewives and invasive predatory cladocerans. Predicted declines in the proportion of small cladocerans were not evident. This study represents the first direct comparison of changes in Lake Ontario Zooplankton production before and after the invasion-induced disruption and will be important to food web-scale investigations of invasion effects.

Lake Ontario zooplankton in 2003 and 2008: Community changes and vertical redistribution

Lake-wide zooplankton surveys are critical for documenting and understanding food web responses to ecosystem change. Surveys in 2003 and 2008 during the binational intensive field year in Lake Ontario found that offshore epilimnetic crustacean zooplankton declined by a factor of 12 (density) and factor of 5 (biomass) in the summer with smaller declines in the fall. These declines coincided with an increase in abundance of Bythotrephes and are likely the result of direct predation by, or behavioral responses to this invasive invertebrate predator. Whole water column zooplankton density also declined from 2003 to 2008 in the summer and fall (factor of 4), but biomass only declined in the fall (factor of 2). The decline in biomass was less than the decline in density because the average size of individual zooplankton increased. This was due to changes in the zooplankton community composition from a cyclopoid/bosminid dominated community in 2003 to a calanoid dominated community in 2008. The increase in calanoid copepods was primarily due to the larger species Limnocalanus macrurus and Leptodiaptomus sicilis. These cold water species were found in and below the thermocline associated with a deep chlorophyll layer. In 2008, most of the zooplankton biomass resided in or below the thermocline during the day. Increased importance of copepods in deeper, colder water may favor Cisco and Rainbow Smelt over Alewife because these species are better adapted to cold temperatures than Alewife.

Lake Ontario water quality during the 2003 and 2008 intensive field years and comparison with long-term trends

Phosphorus loading declined between the 1970s and the 1990s, leading to oligotrophication of the offshore waters of Lake Ontario during that time period. Using lake-wide data from the intensive field years of 2003 and 2008 and from available long-term data sets on several trophic state indicators (total phosphorus [TP], soluble reactive silica [SRSi], chlorophyll a and Secchi disc transparency [SDT]), we tested the hypothesis that oligotrophication of the offshore waters of Lake Ontario has continued in the 2000s. Significant differences between 2003 and 2008 include higher spring (April) TP, SRSi, and SDT in 2008, lower summer (July–August) SDT in 2008, higher summer chlorophyll a in 2008, and lower fall (September) TP, SRSi, and chlorophyll a in 2008. The decline in SRSi from spring to summer was greater in 2008 than in 2003. Change point and regression analyses on the long-term data revealed no trend in spring TP since 1996, in summer chlorophyll a since 1994, in spring SDT since 1998, in spring SRSi or SRSi decline from spring to summer since 1999, or in summer SDT since 2001. Neither the comparison of the 2003 and 2008 surveys nor the analysis of the long-term data supported our hypothesis of continued oligotrophication of the offshore of Lake Ontario in the 2000s.

Spatial extent and dissipation of the deep chlorophyll layer in Lake Ontario during the Lake Ontario lower foodweb assessment, 2003 and 2008

Increasing water clarity in Lake Ontario has led to a vertical redistribution of phytoplankton and an increased importance of the deep chlorophyll layer in overall primary productivity. We used in situ fluorometer profiles collected in lakewide surveys of Lake Ontario in 2008 to assess the spatial extent and intensity of the deep chlorophyll layer. In situ fluorometer data were corrected with extracted chlorophyll data using paired samples from Lake Ontario collected in August 2008. The deep chlorophyll layer was present offshore during the stratified conditions of late July 2008 with maximum values from 4–13 μg l−1 corrected chlorophyll a at 10 to 17 m depth within the metalimnion. Deep chlorophyll layer was closely associated with the base of the thermocline and a subsurface maximum of dissolved oxygen, indicating the features importance as a growth and productivity maximum. Crucial to the deep chlorophyll layer formation, the photic zone extended deeper than the surface mixed layer in mid-summer. The layer extended through most of the offshore in July 2008, but was not present in the easternmost transect that had a deeper surface mixed layer. By early September 2008, the lakewide deep chlorophyll layer had dissipated. A similar formation and dissipation was observed in the lakewide survey of Lake Ontario in 2003.

Density and Distribution of Amphipods in Oneida Lake, New York, after the Introduction of the Exotic Amphipod Echinogammarus ischnus

Abstract The exotic amphipod Echinogammarus ischnus, first reported in North America from western Lake Erie in 1995, was recorded in Oneida Lake, NY in 2001. Some North American studies have suggested that E. ischnus was replacing native amphipods, but other studies found no evidence for this. We sampled amphipods at six depths (<0.2, 0.6, 1.2, 1.8, 3.0, and >3.8 m) along six transects in Oneida Lake to quantify variation in densities of amphipod species as a function of depth, substrate (cobble with Dreissena and with or without macroalgae, sand with or without Dreissena, and macroalgae or submersed vascular plants) and density of Dreissena, and compared the present amphipod density to the historical record. Four species of amphipods, Gammarus fasciatus, Hyalella azteca, E. ischnus, and Crangonyx sp., were collected from Oneida Lake. Gammarus fasciatus was 9 to 90 times more abundant (mean = 0.09 individuals/cm2) than other amphipod species and was collected on all substrates and at all depths, as was H. azteca. Statistical comparisons were made with non-parametric tests between mean ranks of density of amphipods and Dreissena and the other variables. Mean ranks of density of G. fasciatus were correlated with depth (Spearman rank = 0.28, P < 0.0001), but mean ranks of density of H. azteca were not, and neither species was correlated with mean ranks of density of Dreissena. Mean ranks of density of G. fasciatus were greater on sand with or without macroalgae or submersed vascular plants (SVP) or Dreissena than on cobble with macroalgae and Dreissena (H = 28.2, P < 0.0001). Mean ranks of density of H. azteca were greater on sand with SVP, with or without Dreissena, than on sand with Dreissena and without SVP (H = 21.8, P = 0.0013). Echinogammarus ischnus was collected only in water less than 1.8 m depth and always with Dreissena. Mean ranks of density of E. ischnus were correlated with depth (Spearman rank = -0.29, P < 0.0001) and with Dreissena mean ranks of density (Spearman rank = 0.14, P = 0.01). Mean ranks of density of E. ischnus was greater on cobble with Dreissena than on sand with Dreissena regardless of the presence or absence of macroalgae or SVP (H = 35.4, P < 0.0001). Although E. ischnus is established in the near-shore zone of Oneida Lake, we found no evidence that it will replace the native amphipods G. fasciatus and H. azteca.