Predatory demands of Bythotrephes and Leptodora in Saginaw Bay,Lake Huron

Predatory invertebrates can be a major factor in structuring zooplankton communities. We collected zooplankton and the predatory cladocerans Bythotrephes longimanus and Leptodora kindtii at 4 stations in inner Saginaw Bay, Lake Huron during May–October 2009 and April–November 2010. Production of zooplankton prey was estimated using temperature and biomass based empirical models. Population consumption rates of Bythotrephes and Leptodora were estimated using a combination of conversion efficiency, bioenergetics and experimental based models. Bythotrephes biomass peaked in July of 2009 and 2010, and in 2010, the biomass rebounded in the fall after a late summer decline. Leptodora biomass was generally lower than that of Bythotrephes except in June 2009 and August 2010. Zooplankton biomass, production and composition showed strong seasonal patterns, with biomass more than tripling between May and June each year, largely due to an increase in biomass of Daphnia in June. By contrast, total zooplankton biomass declined by over 70% between June and July owing to declines in Daphnia biomass. Population consumption by predatory cladocerans was a large percentage or even exceeded prey production during July and August in Saginaw Bay regardless of which consumption models were used. This short term (< 2 months) but pronounced predation capacity of predatory cladocerans in Saginaw Bay was mainly due to consumption by Bythotrephes rather than Leptodora. Bythotrephes likely play an important role in food web function and should be accounted for when evaluating the flow of energy within the Laurentian Great Lakes systems.     

Lake Erie Central Basin Total Phosphorus Trend Analysis from 1968 to 1982

The total phosphorus data from 1968 to 1982 in the Lake Erie central basin trend study area was analyzed to determine in-lake responses to the Great Lakes Water Quality Agreement (GLWQA) phosphorus loading reduction program. The available data for each year were divided into five subsets according to time of year and depth of the water column. Each data subset was regressed as a function of time and total phosphorus loadings to Lake Erie. Linear regression analysis indicates that the in-lake phosphorus concentrations have been decreasing and are well correlated with decreased loadings to the lake. The highest rate of phosphorus decrease with time (0.56 ± 0.10 mg · m⁻³ yr⁻¹) was obtained by using epilimnetic concentrations from April to December for each year. This data subset also shows the best correlation with decreasing phosphorus loadings. From 1968 to 1982, Lake Erie offshore phosphorus concentrations responded to decreasing external phosphorus loadings at a rate of 0.45 ± 0.09 mg · m⁻³ per thousand metric tonnes.     

The influence of light and nutrients on benthic filamentous algal growth: A case study of Saginaw Bay,Lake Huron

The benthic filamentous green algal (FGA) community of Saginaw Bay, Lake Huron, has not been extensively examined, despite its apparent link to shoreline fouling. The objective of this study was to elucidate factors, including light, nutrients, and substrate, expected to affect growth of FGA. We examined the FGA community in the southwest region of the bay, which started ~ 5 km northwest from the Saginaw River, a known source of nutrients and sediment. Cellular nutrient quotas and photosynthetic parameters were measured in FGA over a range of water column depths and distances from the Saginaw River and then compared to published thresholds of limitation. Our results suggest that light limitation sets the maximum depth of growth and prevents growth near the Saginaw River. Our data also indicate that a light gradient extends from the Saginaw River into our study area, but we did not observe direct evidence for the existence of a parallel nutrient gradient. Most of the FGA community experienced both light and P stress, with the exception of 20% of the sampled FGA, which experienced saturating levels of midday light. Nitrogen deficiency was never observed. Our results suggest that post-dreissenid invasion increases in water clarity extended the maximum depth limit for FGA from ~ 3.3 to ~ 5 m, greatly increasing the area of FGA growth. One quarter of sampled FGA were strictly P-limited and 87–100% of sampled FGA were P-deficient, suggesting that phosphorus-based management approaches could successfully reduce FGA growth in inner Saginaw Bay.     

Implications of hypoxia on the North Branch of the Kawkawlin River

The North Branch of the Kawkawlin River, a Saginaw Bay tributary, is frequently in a state of non-attainment of the Michigan standard for dissolved oxygen (5 mg/L), as documented in a Total Maximum Daily Load issued by the Michigan Department of Environmental Quality in 2007. Analysis of a reach of the North Branch of the Kawkawlin River during the summers of 2011 and 2012 revealed conditions that can adversely affect fish habitat and water quality. Dissolved oxygen levels were found to be below 2 mg/L during summer months, which is too low to sustain warm water fish species. Furthermore, benthic invertebrates were nearly nonexistent in the hypoxic reach of the river. On average, water samples from the river channel near a riparian wetland in the hypoxic reach exhibited unusually low pH (~ 7), low D.O. (< 1 mg/L), elevated phosphorus, and reduced turbidity compared to upstream samples. The hypoxia and low pH in the riparian wetland were consistent with conditions able to reduce sedimentary iron thereby mobilizing bound reactive phosphorus (phosphate-P).     

Trends in the distribution and abundance of Hexagenia spp. in Saginaw Bay,Lake Huron, 1954–2012: Moving towards recovery?

Since at least the 1940s, multiple anthropogenic disturbances to the Laurentian Great Lakes have had detrimental effects on benthic habitats and biota including decimating the environmentally sensitive burrowing mayfly genus Hexagenia around the mid-1950s. While remediation efforts have facilitated recovery of some populations, benthic surveys in Saginaw Bay, Lake Huron in the last 50 years have only occasionally discovered Hexagenia nymphs. Recently, adult Hexagenia swarms have been reported near the bay; therefore, we corroborated the local presence of Hexagenia adults and evaluated the current status of Saginaw Bay Hexagenia nymphs. We quantified adults during mayfly emergence events in 2010 at three Tawas City, Michigan, USA area locations, and found > 17 Hexagenia/m²/site. We quantified nymphs from Ponar grab samples collected at 57 sites in Saginaw Bay between 2009 and 2012, and found 1.5 nymphs/m² overall with nymphs present at 15.8% of sites sampled, their greatest documented distribution in Saginaw Bay since 1956. Additionally, we mapped bay sediment composition and related sampling site abiotic conditions with both Hexagenia presence and abundance using Zero-Inflated Poisson regression. Model results indicate that the probability of observed Hexagenia absence being true absence is positively related to both sediment sandiness and surficial dissolved oxygen concentration while Hexagenia abundance is greatest where surficial temperatures are ~ 18.6 °C and is also related to sediment sand content. The documentation of nearby adults and in bay nymphs may indicate the beginning of a Hexagenia return to Saginaw Bay, and, therefore, a possible improvement of the ecosystem's benthic health.     

Microzooplankton distribution,dynamics, and trophic interactions relative to phytoplankton and quagga mussels in Saginaw Bay,Lake Huron

Invasive quagga mussels have recently replaced zebra mussels as the dominant filter-feeding bivalves in the Great Lakes. This study examined microzooplankton (i.e., grazers < 200 μm) and their trophic interactions with phytoplankton, bacteria, and bivalve mussels in Saginaw Bay, Lake Huron, following the zebra to quagga mussel shift. Microzooplankton distribution displayed strong spatial and temporal variability (1.73–28.5 μg C/L) relative to phytoplankton distribution. Ciliates were the dominant component, especially in the spring and early summer. Rotifers and dinoflagellates increased toward late summer/fall in the inner and outer parts of the bay, respectively. Microzooplankton grazing matched bacterial growth rates and removed ca. 30% of the phytoplankton standing stock in the < 100 μm size fraction per day. The greatest herbivory occurred at the site dominated by colonial cyanobacteria. Microzooplankton, which comprised < 4% of the quagga mussels prey field (i.e. available prey), contributed 77% and 34% to the quagga carbon-based diet during Microcystis and diatom blooms, respectively. Feeding on microzooplankton could buffer mussels during lean periods, or supplement other consumed resources, particularly during noxious cyanobacterial blooms. The results of this study demonstrate that microzooplankton are a resilient and critical component of the Saginaw Bay ecosystem.     

Increased Abundance and Depth of Submersed Macrophytes in Response to Decreased Turbidity in Saginaw Bay,Lake Huron

Submersed macrophyte communities and turbidity near shore were measured from 1991 to 1993 to determine if more light resulting from colonization of zebra mussels (Dreissena polymorpha Pallas) into Saginaw Bay in 1990 corresponded with changes in macrophyte distribution. Turbidity was sampled along five transects distributed at intervals perpendicular to the perimeter of inner Saginaw Bay to monitor changes in light available to plants in Saginaw Bay. Vegetation was sampled in July along these transects to determine the distribution and composition of the macrophyte communities each summer. We also measured the maximum depth of colonization and the area of plant coverage by use of 31 transects evenly distributed around Saginaw Bay in August. Turbidity decreased (P ≤ 0.097) at transects in northern littoral regions from 1991 to 1993 over submersed plant communities and uncolonized sediments, but not in southern littoral regions. The relative abundance of submersed macrophytes increased (P ≤ 0.0001) at all transects from 1991 to 1993, especially at transects where turbidity decreased significantly. Maximum depth of colonization (2.0 m) and the area of macrophyte coverage (101.3 km²) increased in Saginaw Bay, especially in the northwestern littoral region of the bay. Macrophytic chlorophytes, charophytes, and Vallisneria americana increased (P ≤ 0.003) in relative abundance most at transects where turbidity decreased significantly. These results demonstrate that even in a large well-mixed lacustrine environment, zebra mussels have the capacity to reduce turbidity sufficiently to allow submersed macrophytes to expand their distribution and abundance.     

Carbonate Equilibria and the Distribution of Inorganic Carbon in Saginaw Bay

The spatial and temporal distribution of selected inorganic carbon equilibrium species is presented for eleven locations, representing the near surface waters of Saginaw Bay in Lake Huron during 1974. Carbon dioxide and calcite equilibrium conditions are determined through solution of temperature and ionic strength adjusted equilibria and evaluated with respect to suitability as indicators of physical conditions and biological activity in the waters of Saginaw Bay. The participation of inorganic carbon species in several chemical and biochemical reactions leads to the utility of these species in reflecting documented spatial and temporal trends in phytoplankton biomass, temperature, ionic strength, and ionic composition. Substantial seasonal variations in carbon dioxide and calcite equilibrium conditions were observed at several locations throughout the bay. The magnitude of variation was greatest in the more productive areas of the bay, where extensive calcite supersaturation and carbon dioxide undersaturation occurred. Maximum calcite saturation and minimum carbon dioxide saturation were calculated for the warm, productive summer months. Transformations in chemical equilibria were mediated by both physical and biochemical factors,as reflected by seasonal changes in temperature and pH. The significance of photosynthetic activity was most pronounced in the inner bay and in shore zones, while temperature became of greater relative importance where bay waters mixed with the open waters of Lake Huron. Strong correlations between time averaged data for chlorophyll a and carbon dioxide (R = 0.97) and chlorophyll a and calcite saturation (R = 0.95) indicate the importance of photosynthetic activity in establishing the distribution of equilibrium conditions of inorganic carbon species.     

A status assessment and review of the herpetofauna within the Saginaw Bay of Lake Huron

Amphibians and reptiles are key bioindicators of environmental health and habitat quality and can be used to provide baseline information to help assess habitat conditions and evaluate restoration success. In 2011 and 2012 we conducted comprehensive herpetological surveys throughout the Saginaw Bay area and assessed community composition, species richness, and spatial distribution. We also compared current distributions to historic observations and habitat conditions. A total of 25 taxa (13 species of reptiles and 12 species of amphibians) were observed within the Saginaw Bay area during this study. Herpetofauna were conspicuously unobserved in areas where Phragmites australis dominates the vegetation community and were concentrated in remaining areas of suitable habitat adjacent to Phragmites. Herpetofauna observations were clustered in areas where Phragmites and other invasive plant species were rare or absent. We were able to relate categorization of Phragmites invasion (i.e., 1 = 0–10% colonization detected, 2 = 10–50%, 3 > 50%) to both a biologically and statistically significant decrease in amphibian and reptile species richness. Our results indicate that Saginaw Bay can support a diverse herpetofauna community and there is potential to restore and improve this region for rare and common amphibian and reptile species. Removal of invasive plant species would greatly improve herpetofaunal communities within Saginaw Bay. Our results will help this region's resource professionals assess the quality of habitat and set goals for restoration of amphibian and reptile habitats.     

Cladophora,mass transport,and the nearshore phosphorus shunt

The nearshore phosphorus shunt hypothesis and the potential for mussels to excrete phosphorus sufficient to meet the growth requirements of Cladophora are now well accepted by scientists studying Great Lakes biogeochemistry. The response of algal growth to near bottom water column phosphorus concentrations and the interplay between excretion and mass transport in yielding those concentrations have, however, not been elucidated. Here we present soluble reactive phosphorus profiles from the near bottom environment of Lake Michigan at a site near Good Harbor Bay, Michigan, where both mussels and Cladophora were present. Soluble reactive phosphorus was observed to accumulate under quiescent conditions, establishing a concentration boundary layer (CBL), 5–15 cm thick, with near bottom concentrations on the order of 2–8 μg P/L. A one-dimensional model was applied to determine mass transport conditions mediating the transition from CBL formation to CBL destruction. Significant wave height (SWH) was used as an indicator of mass transport intensity, and it was determined that the formation/destruction transition occurred at a SWH of 0.2 m at the 8-m study site depth. The Great Lakes Cladophora Model was applied to determine the time intervals required to saturate (1 day with the CBL present) and deplete (14 days with the CBL absent) algal internal P stores. A review of SWH conditions at the study site indicated that a CBL would be expected to form at a frequency sufficient to support the phosphorus nutrition of Cladophora over the entire May to August interval.     

Diatom-based Weighted-averaging Transfer Functions for Great Lakes Coastal Water Quality: Relationships to Watershed Characteristics

In an effort to develop indicators for Great Lakes near-shore conditions, diatom-based transfer functions to infer water quality variables were developed from 155 samples collected from coastal Great Lakes wetlands, embayments and high-energy shoreline sites. Over 2,000 diatom taxa were identified, and 352 taxa were sufficiently abundant to include in transfer function development. Multivariate data exploration revealed strong responses of the diatom assemblages to stressor variables, including total phosphorus (TP). Spatial variables such as lake, latitude and longitude also had notable relationships with assemblage characteristics. A diatom inference transfer function for TP provided a robust reconstructive relationship (r² = 0.67; RMSE = 0.28 log(μg/L); r²jackknife = 0.55; RMSEP = 0.33 log (μg/L)) that improved following the removal of 13 samples that had poor observed-inferred TP relationships (r² = 0.75; RMSE = 0.22 log(μg/L); r²_jackknife = 0.65; RMSEP = 0.26 log (μg/L)). Diatom-based transfer functions for other water quality variables, such as total nitrogen, chloride, and chlorophyll a also performed well. Measured and diatom-inferred water quality data were regressed against watershed characteristics (including gradients of agriculture, atmospheric deposition, and industrial facilities) to determine the relative strength of measured and diatom-inferred data to identify watershed stressor influences. With the exception of pH, diatom-inferred water quality variables were better predicted by watershed characteristics than were measured water quality variables. Because diatom communities are subject to the prevailing water quality in the Great Lakes coastal environment, it appears they can better integrate water quality information than snapshot measurements. These results strongly support the use of diatoms in Great Lakes coastal monitoring programs.     

Great Lakes total phosphorus revisited: 1. Loading analysis and update (1994–2008)

Phosphorus load estimates have been updated for all of the Great Lakes with an emphasis on lakes Superior, Michigan, Huron and Ontario for 1994–2008. Lake Erie phosphorus loads have been kept current with previous work and for completeness are reported here. A combination of modeling and data analysis is employed to evaluate whether target loads established by the Great Lakes Water Quality Agreement (GLWQA, 1978, Annex 3) have been and are currently being met. Data from federal, state, and provincial agencies were assembled and processed to yield annual estimates for all lakes and sources. A mass-balance model was used to check the consistency of loads and to estimate interlake transport. The analysis suggests that the GLWQA target loads have been consistently met for the main bodies of lakes Superior, Michigan and Huron. However, exceedances still persist for Saginaw Bay. For lakes Erie and Ontario, loadings are currently estimated to be at or just under the target (with some notable exceptions). Because interannual variability is high, the target loads have not been met consistently for the lower Great Lakes. The analysis also indicates that, because of decreasing TP concentrations in the lakes, interlake transport of TP has declined significantly since the mid-1970s. Thus, it is important that these changes be included in future assessments of compliance with TP load targets. Finally, detailed tables of the yearly (1994–2008) estimates are provided, as well as annual summaries by lake tributary basin (in Supplementary Information).     

Community engagement and the importance of partnerships within the Great Lakes Areas of Concern program: A mixed-methods case study

The Great Lakes Areas of Concern (AOC) program was created through amendments to the Great Lakes Water Quality Agreement (GLWQA) in 1987 to restore contaminated sites using an ecosystem-based approach. This program represents one of the first instances of ecosystem-based management (EBM) in the Great Lakes region with a specific focus on the inclusion of the public and local stakeholders in the process. Despite official language incorporating EBM in the AOC program, implementation of these practices has not been consistent across AOCs given differences in local arrangements of Public Advisory Councils (PACs), approaches to community engagement, and environmental problems. To better understand community engagement in these complex AOCs, this research investigated community, PAC, and state agency perspectives in three AOCs in Michigan: the Kalamazoo River, Saginaw River and Bay, and Rouge River AOCs. We gathered data through interviews, focus groups, and participatory observations with community members, PAC members, and state officials in each AOC. Findings indicate that communities in these areas have minimal connection to the AOC program and PACs. Community members tended to have greater connection to local organizations that provide a variety of opportunities for community members to engage with their environment in ways they value. To better connect the public to the AOC program, PACs may benefit from intentional partnerships with community organizations to increase community engagement. To consistently bolster community engagement in AOCs, we further recommend that state agencies provide additional resources to improve connection to local communities.     

Contaminant Management Strategies for the Great Lakes: Optimal Solutions Under Uncertain Conditions

Optimization, uncertainty analysis, and mass balance modeling techniques were combined into a framework that can help decision makers identify cost-effective load reduction methods for achieving acceptable contaminant concentrations in the Great Lakes. The utility of the framework is demonstrated by deriving an optimal phosphorus load reduction plan for the Great Lakes. An optimal plan is defined as the least-cost approach that can achieve desired phosphorus concentrations in all Great Lakes basins under realistic, stochastic phosphorus loading and settling rates. The analysis suggests that implementation of phosphorus load reduction measures recommended in the U. S. - Canadian 1978 Great Lakes Water Quality Agreement, its 1983 supplement, and other plans that do not account for environmental uncertainty may by sub-optimal. Compared with the load reduction strategies of the 1978 Water Quality Agreement and its supplement, implementation of the optimized load reduction strategy would lead to substantial annual cost savings and an increased probability of achieving desired phosphorus concentrations. Results emphasize the importance of quantitatively accounting for environmental uncertainty in management models.     

Researcher disciplines and the assessment techniques used to evaluate Laurentian Great Lakes coastal ecosystems

The Laurentian Great Lakes of North America have been a focus of environmental and ecosystem research since the Great Lakes Water Quality Agreement in 1972. This study provides a review of scientific literature directed at the assessment of Laurentian Great Lakes coastal ecosystems. Our aim was to understand the methods employed to quantify disturbance and ecosystem quality within Laurentian Great Lakes coastal ecosystems within the last 20 years. We focused specifically on evidence of multidisciplinary articles, in authorship or types of assessment parameters used. We sought to uncover: 1) where Laurentian Great Lakes coastal ecosystems are investigated, 2) how patterns in the disciplines of researchers have shifted over time, 3) how measured parameters differed among disciplines, and 4) which parameters were used most often. Results indicate research was conducted almost evenly across the five Laurentian Great Lakes and that publication of coastal ecosystems studies increased dramatically ten years after the first State of the Great Lakes Ecosystem Conference in 1994. Research authored by environmental scientists and by multiple disciplines (multidisciplinary) have become more prevalent since 2003. This study supports the likelihood that communication and knowledge-sharing is happening between disciplines on some level. Multidisciplinary or environmental science articles were the most inclusive of parameters from different disciplines, but every discipline seemed to include chemical parameters less often than biota, physical, and spatial parameters. There is a need for an increased understanding of minor nutrient, toxin, and heavy metal impacts and use of spatial metrics in Laurentian Great Lakes coastal ecosystems.     

Spatial and temporal patterns of macroscopic benthic primary producers in Saginaw Bay,Lake Huron

We investigated spatial and temporal patterns in macroscopic benthic primary producer biomass, production, and composition in inner Saginaw Bay in 2009 and 2010. Charophytes and filamentous algae (FA) were relatively abundant, and vascular macrophytes were less common. The probability of benthic primary producer presence increased with the proportion of benthic substrate composed of rock. Most benthic primary producer biomass occurred at depths of 2–4 m, with very little biomass observed beyond 4 m deep. Charophyte and vascular macrophyte abundances displayed consistent patterns related to distance from the mouth of the Saginaw River. FA abundance also displayed such patterns, but they reversed between 2009 and 2010. Macrophytic benthic primary producer communities were generally dominated by charophytes. Three genera of vascular macrophytes, including Myriophyllum, were also observed. Filamentous algal communities were composed of a mixture of FA taxa. Ten FA genera were observed, including the red alga Compsopogon. Dominance of Compsopogon was related to low water clarity and low TP. Biomass-based benthic production estimates indicated that charophytes and FA strongly dominated macroscopic benthic production; production of vascular macrophytes was relatively low. The observed relationships of abundance and environmental conditions suggested regulation of benthic producer biomass by a shifting mosaic of substratum, nutrient, and light availabilities. The diverse nature of the benthic producer community could complicate understanding and management of excess benthic biomass and beach fouling in Saginaw Bay.     

Quantifying the predatory effect of round goby on Saginaw Bay dreissenids

Invasive dreissenid mussels (D. polymorpha and D. r. bugensis) have fundamentally altered Laurentian Great Lake ecosystems, however in many areas their abundances have declined since the mid-1990s. Another invader, the benthic fish round goby (Neogobius melanostomus), is morphologically adapted to feed on dreissenids and likely affects dreissenid populations; however, the degree of this predatory effect is variable. In 2009 and 2010, we examined round goby abundances, size distributions, diet contents, and diet selectivity in Saginaw Bay, Lake Huron; a shallow bay that has been subjected to numerous anthropogenic stressors. We further used a consumption model to estimate dreissenid consumption by three different size classes of round goby. Round gobies were found throughout the bay and most were smaller than 80 mm total length. Round gobies of all sizes consumed dreissenids (including fish as small as 30 mm total length), though dreissenids were rarely preferred. The relative proportion of dreissenids (by biomass) present in diets of round gobies increased with fish size, but also throughout the year for all size classes. Despite this, overall consumptive effects of round gobies on dreissenids in Saginaw Bay were low. Many dreissenids present in the bay were larger than those consumed by round gobies. Bioenergetics-based model estimates suggest that the smallest round gobies are responsible for the majority of dreissenid consumption. While our findings are limited to soft substrates and influenced by sampling restrictions, our study design allowed us to put bounds on our estimates based upon these multiple sources of uncertainty.     

Effects of Zebra Mussels (Dreissena polymorpha) on Bacterioplankton: Evidence for Both Size-Selective Consumption and Growth Stimulation

Zebra mussels had significant direct and possible indirect effects on heterotrophic bacteria in two contrasting sites in Saginaw Bay. At a eutrophic site in the inner portion of Saginaw Bay, mussels fed directly on bacterial-sized particles and had a negative impact on bacterial abundances. Mussels removed large bacteria (> 0.9 μm) more effectively than small bacteria at this site. Individual mussels cleared from 37–89 ml per day. Results using different sizes of fluorescent microspheres suggest that zebra mussels have a lower limit for particle size removal that is less than 0.4 μm. Contradictory to inner bay results, mussels at an outer bay oligotrophic site had a positive impact on heterotrophic bacterial abundance, perhaps as a result of indirect effects, such as nutrient or organic carbon excretion by the mussels. Differences in the impact of mussels on the bacterial communities of the inner bay and outer bay probably result from differences in trophic state and bacterial community structure. A hypothesized smaller size of bacteria at outer bay sites may enable them to escape heavy predation pressure from mussels and the high rates of mussel nutrient excretion may facilitate their growth in these nutrient depleted conditions.     

Mussel-derived stimulation of benthic filamentous algae: The importance of nutrients and spatial scale

The reoccurrence of benthic filamentous algal (FA) blooms in the Great Lakes, without associated increases in phosphorus loading, has stimulated renewed interest in determining the causes of Great Lakes benthic algal blooms. We investigated the potential roles of invasive mussels and nutrient limitation with experimental substrata within inner Saginaw Bay. FA abundance on live mussel substrata was typically significantly greater than that on inert (empty shell or rock) substrata. Nutrient addition (from an artificial source) significantly increased FA abundance on inert substrata. These results suggest that: 1) mussel nutrient excretion could be a primary stimulatory mechanism; 2) mussel-mediated stimulation may be even stronger in other, more oligotrophic, Great Lakes nearshore zones; and 3) increased nutrient loading to inner Saginaw Bay may exacerbate existing FA blooms. FA abundance on inert substrata was not affected, even in close proximity to mussels, indicating that the observed stimulatory effect of mussel-derived P on live mussels attenuated at very small spatial scales, on the order of centimeters or less.     

Age and growth of round gobies in Lake Michigan,with preliminary mortality estimation

The round goby (Neogobius melanostomus) is a prevalent invasive species throughout Lake Michigan, as well as other Laurentian Great Lakes, yet little information is available on spatial variation in round goby growth within one body of water. Age and growth of round goby at three areas of Lake Michigan were studied by otolith analysis from a sample of 659 specimens collected from 2008 to 2012. Total length (TL) ranged from 48 to 131 mm for Sturgeon Bay, from 50 to 125 mm for Waukegan, and from 54 to 129 mm for Sleeping Bear Dunes. Ages ranged from 2 to 7 years for Sturgeon Bay, from 2 to 5 years for Waukegan, and from 2 to 6 years for Sleeping Bear Dunes. Area-specific and sex-specific body–otolith relationships were used to back-calculate estimates of total length at age, which were fitted to von Bertalanffy models to estimate growth rates. For both sexes, round gobies at Sleeping Bear Dunes and Waukegan grew significantly faster than those at Sturgeon Bay. However, round goby growth did not significantly differ between Sleeping Bear Dunes and Waukegan for either sex. At all three areas of Lake Michigan, males grew significantly faster than females. Based on catch curve analysis, estimates of annual mortality rates ranged from 0.79 to 0.84. These relatively high mortality rates suggested that round gobies may be under predatory control in Lake Michigan.