Scott N. Higgins

What a difference a species makes: a meta–analysis of dreissenid mussel impacts on freshwater ecosystems

We performed a meta-analysis of published studies and long-term monitoring data sets to evaluate the effects of dreissenid mussels (Dreissena polymorpha and D. rostriformis bugensis), two of the worlds most problematic biological invaders, on the biogeochemistry, flora, and fauna of lakes and rivers across North America and Eurasia. Dreissenid effects were structured along two distinct energy pathways. For the pelagic–profundal pathway, large mean reductions in phytoplankton (−35% to −78%) and zooplankton (−40% to −77%) biomass occurred and were dependent on habitat type. The largest effects were found in rivers, followed by littoral and pelagic habitats in lakes. In contrast, benthic energy pathways within littoral habitats of lakes and rivers showed dramatic increases in mean benthic algal and macrophyte biomass (+170% to +180%), sediment-associated bacteria (about +2000%), non-dreissenid zoobenthic biomass (+160% to +210%), and total zoobenthic biomass, which includes dreissenid mussel soft tissues (+...

AN ECOLOGICAL REVIEW OF CLADOPHORA GLOMERATA (CHLOROPHYTA) IN THE LAURENTIAN GREAT LAKES1

Cladophora glomerata (L.) Kütz. is, potentially, the most widely distributed macroalga throughout the world’s freshwater ecosystems. C. glomerata has been described throughout North America, Europe, the Atlantic Islands, the Caribbean Islands, Asia, Africa, Australia and New Zealand, and the Pacific Islands. Cladophora blooms were a common feature of the lower North American Great Lakes (Erie, Michigan, Ontario) from the 1950s through the early 1980s and were largely eradicated through the implementation of a multibillion‐dollar phosphorus (P) abatement program. The return of widespread blooms in these lakes since the mid‐1990s, however, was not associated with increases in P loading. Instead, current evidence indicates that the resurgence in blooms was directly related to ecosystem level changes in substratum availability, water clarity, and P recycling associated with the establishment of dense colonies of invasive dreissenid mussels. These results support the hypothesis that dreissenid mussel invasions may induce dramatic shifts in energy and nutrient flow from pelagic zones to the benthic zone.

Great Lakes Cladophora in the 21st Century: Same Algae-Different Ecosystem

ABSTRACT Nuisance growth of the attached, green alga Cladophora was considered to have been abated by phosphorus management programs mandated under the Great Lakes Water Quality Agreement. The apparent resurgence of nuisance growth in Lakes Erie, Michigan and Ontario has been linked conceptually to ecosystem alterations engineered by invasive dreissenid mussels (Dreissena polymorpha and Dreissena bugensis). Here, we apply contemporary modeling tools and historical water quality data sets in quantifying the impact of long-term changes in phosphorus loading and dreissenid-mediated changes in water clarity on the distribution and production of Cladophora. It is concluded that reductions in phosphorus loading in the predreissenid period achieved the desired effect, as model simulations were consistent with the biomass declines reported from the early 1970s to the early 1980s. These declines were, however, largely offset by dreissenid-driven changes in water clarity that extended the depth of colonization by Cladophora, increasing total production. We were not able to isolate and quantify the significance of dreissenid mediation of phosphorus cycling using the historical database. Phosphorus management remains the appropriate mechanism for reducing nuisance levels of Cladophora growth. The development of action plans will require an improved understanding of nearshore phosphorus dynamics such as might be obtained through regular monitoring of soluble reactive phosphorus levels, internal phosphorus content and Cladophora biomass in impacted nearshore regions of the Great Lakes.

A Pound of Prevention, Plus a Pound of Cure: Early Detection and Eradication of Invasive Species in the Laurentian Great Lakes

ABSTRACT Ballast water regulations implemented in the early 1990s appear not to have slowed the rate of new aquatic invasive species (AIS) establishment in the Great Lakes. With more invasive species on the horizon, we examine the question of whether eradication of AIS is a viable management strategy for the Laurentian Great Lakes, and what a coordinated AIS early detection and eradication program would entail. In-lake monitoring would be conducted to assess the effectiveness of regulations aimed at stopping new AIS, and to maximize the likelihood of early detection of new invaders. Monitoring would be focused on detecting the most probable invaders, the most invasion-prone habitats, and the species most conducive to eradication. When a new non-native species is discovered, an eradication assessment would be conducted and used to guide the management response. In light of high uncertainty, management decisions must be robust to a range of impact and control scenarios. Though prevention should continue to be the cornerstone of management efforts, we believe that a coordinated early detection and eradication program is warranted if the Great Lakes management community and stakeholders are serious about reducing undesired impacts stemming from new AIS in the Great Lakes. Development of such a program is an opportunity for the Laurentian Great Lakes resource management community to demonstrate global leadership in invasive species management.

The Wall of Green: The Status of Cladophora glomerata on the Northern Shores of Lake Erie's Eastern Basin, 1995–2002

The biomass, areal coverage, algal bed characteristics, and tissue phosphorus concentrations of Cladophora glomerata were measured at 24 near shore rocky sites along the northern shoreline of Lake Eries eastern basin between 1995–2002. Midsummer areal coverage at shallow depths (≤ 5 m) ranged from 4–100%, with a median value of 96%. Cladophora biomass began accumulating at most sites during early May, and achieved maximum values by mid-July. Peak seasonal biomass ranged from < 1 to 940 g/m2 dry mass (DM), with a median value of 171 g/m2 DM. Nearshore water concentrations of total phosphorus (TP) were lower than during pre-phosphorus abatement years. However, Cladophora biomass levels were similar to reported values in those years. The midsummer “die off” occurred shortly after the biomass peak, when water temperatures neared 22.5°C. Areal coverage declined after die-off to < 10%, mean filament lengths declined from 33 cm to < 1 cm, and mean biomass declined to < 1 g DM/m2. Tissue phosphorus varied seasonally, with initial high values in early May (0.15 to 0.27% DM; median 0.23% DM) to midsummer seasonal low values during peak biomass (0.03 to 0.23% DM; median 0.06% DM). Cladophora biomass is sensitive to changes in phosphorus and light availability, and reductions in biomass previously achieved through phosphorus control may now be reversed because of increased water transparency and phosphorus availability to the benthos following establishment of dreissenids.

Environmental Controls of Cladophora Growth Dynamics in Eastern Lake Erie: Application of the Cladophora Growth Model (CGM)

ABSTRACT The Cladophora growth model (CGM) was used to estimate the importance of light, temperature, phosphorus, and self-shading on the spatial and temporal variability of Cladophora growth rates and biomass accrual in eastern Lake Erie during 2002. The CGM predicted that Cladophora growth was highly sensitive to spatial and temporal variations in soluble phosphorous concentration (SRP). Specifically the CGM predicted that: 1) Site-to-site differences in SRP concentration resulted in a 2×difference in depth-integrated biomass; 2) maximum growth rates were strongly influenced by SRP concentrations during periods of rapid biomass accrual (mid-June to mid-July); 3) inter-annual differences in SRP concentration during the spring period (∼ 1 μg/L) could result in up to a 3.5×difference in depth integrated biomass; 4) Spatial variations in water clarity could result in a 2×difference in depth-integrated biomass between sites, with variations betweens sites occurring primarily between 2–6 m depth; 5) the midsummer sloughing phenomenon likely resulted from self-shading by the algal canopy; and 6) the seasonal growth pattern of Cladophora was strongly regulated by temperature.

Modeling the Growth, Biomass, and Tissue Phosphorus Concentration of Cladophora glomerata in Eastern Lake Erie: Model Description and Field Testing

Cladophora glomerata is a filamentous alga that currently forms extensive blooms in nearshore areas of Lake Ontario, eastern Lake Erie, Lake Michigan, and isolated locations in Lake Huron. During the late 1970s an extensive effort was put forward to model Cladophora growth and biomass accrual based on several highly dynamic ecological variables including: Photosynthetically active radiation (PAR), soluble reactive phosphorus (SRP), water temperature, and carrying capacity (Canale and Auer 1982a). The original “Canale and Auer” model was developed and validated in proximity to a sewage treatment outfall in Lake Huron and predicted Cladophora growth and biomass over a range of SRP concentrations, at shallow depths (0–3 m), with reasonable accuracy. We present a revised version of the “Canale and Auer” model, which we refer to as the Cladophora Growth Model (CGM). The CGM expands its utility of the Canale and Auer model to greater depths and to areas of non-point source P loading while reducing the quantity of input data required. The CGM was incorporated into a computer simulation model using Stella modeling software, and is available from the corresponding author of this manuscript. The CGM was tested over a single growing season at five sites, and three depths (2, 5, 10 m), that represented a wide geographical distribution and expected range in ecological conditions in eastern Lake Erie. The CGM predicted growth, biomass, and tissue phosphorus concentrations with reasonable accuracy. The revised model is useful for: 1) Predicting Cladophora growth, biomass, and tissue phosphorus concentrations under non-point source P loading with no depth restrictions; 2) providing estimates of the timing and magnitude of the midsummer sloughing phenomenon; 3) determining the contribution of Dreissena invasion to the resurgence of Cladophora in eastern Lake Erie; and 4) developing management strategies for Cladophora abatement.

Primary Production and Carbon Dioxide Metabolic Balance of a Lake-Rich Arctic River Floodplain: Partitioning of Phytoplankton, Epipelon, Macrophyte, and Epiphyton Production Among Lakes on the Mackenzie Delta

The Mackenzie River Delta, a floodplain system in the western Canadian Arctic contains approximately 45,000 lakes used by resident and migratory fish, aquatic birds, and mammal populations, which are critical resources for aboriginal peoples. Our results show the Mackenzie Delta is a remarkably productive aquatic ecosystem, not out of place relative to other large river floodplains and unusually productive for its Arctic latitude. Along with other such deltas of north flowing rivers to the Arctic Ocean, it represents a critical habitat of high productivity to support dependent consumers. Our results also showed a consistent under-saturation of CO2 in lake waters of the delta, co-occurring even with high concentrations of dissolved organic carbon, and indicating P:R greater than 1 during the open water period. In less turbid lakes, abundant macrophytes provide a considerable surface area for supporting production of epiphyton, but epiphyton production is constrained strongly by macrophyte shading, when macrophyte biomass is high. Despite this, epiphyton represented a greater supply of non-macrophyte algal carbon than phytoplankton, and thus explains why benthic algae may be a more important food source for primary consumers than phytoplankton, except in the most turbid systems. Most importantly, the high autotrophic production in the Delta lakes relative to nearby lakes on the arctic tundra suggests the extended ice-free season of the floodplain lakes and their landscape setting on, and replenishment by, nutrient-rich river sediments, is the strongest influence on aquatic production levels.

The effect of dreissenid invasions on chlorophyll and the chlorophyll:total phosphorus ratio in north-temperate lakes

We investigated the effects of dreissenid mussel (Dreissena polymorpha and D. rostriformis bugensis) invasions on the concentrations of chlorophyll a (Chl) and total phosphorus(TP), and the Chl:TP ratio within 27 north-temperate lakes that spanned large gradients in lake size and trophic status, using two approaches: (i) regression analysis and (ii) hierarchical modeling. Overall, Chl declined by 40%–45% after dreissenid invasion and the magnitude of effect was structured by epilimnetic volume within stratified, but not nonstratified (mixed) lakes. Declines in TP over the invasion period were significant only for stratified systems and were smaller (∼16% reduction from pre-invasion values) than for Chl. Across the complete trophic gradient (i.e., among lakes) the regression approach (analysis of covariance) indicated a significant decline in the intercept of the Chl:TP, but not the slope. In contrast, hierarchical modeling indicated a decline in the slope of Chl:TP among lakes between pre- and post-invasi...

Planktonic Primary Production in the Offshore Waters of Dreissenid-infested Lake Erie in 1997

Phosphorus and chlorophyll a (Chl a) concentrations in 1997 decreased from the west basin, which receives most of the lakes nutrient loading, to the central and eastern basins. By contrast, average areal primary production varied little between basins, because higher Chl a concentrations in the west basin were offset by poorer light penetration. Sub-epilimnetic production in the deeper central and east basins, which might be favored by trends to greater water clarity over recent decades, was a minor contributor to total primary production. With additional data from the literature, it was shown that pre-dreissenid Lake Erie produced significantly less Chl a relative to total phosphorus (TP) than did other Great Lakes or smaller inland lakes. Chl a:TP ratios did not change significantly consequent to dreissenid colonization. Despite the continued low yield of Chl a, Lake Erie in post-dreissenid years had areal primary production relative to TP as high as other, dreissenid-free, Great Lake locations and much higher than comparably oligotrophic inland lakes. Absolute concentrations of TP and Chl a have decreased over the decades of nutrient controls and, in the west basin, in parallel with dreissenid colonization. The lake nonetheless remained highly efficient at translating TP into primary production up to 1997, revealing little or none of the depression associated with dreissenid impacts in shallower systems.