Additions of Nutrients and Major Ions by the Atmosphere and Tributaries to Nearshore Waters of Northwestern Lake Huron

Nutrient additions by the atmosphere and six tributaries to nearshore waters of northwestern Lake Huron were measured at weekly intervals from August 1975 to July 1976. The atmosphere contributed 43% of the nitrogen (N) and 10% of the phosphorus (P) that was added during the year. The 1975–76 atmospheric loading rate of total N to this area (11 kg/ha/yr) was one of the highest found to date in the United States. N was conserved more efficiently than P in the tributary drainage basins. Of the N and P that fell annually on the watersheds under study, 2 to 37% of the N and 31 to 84% of the P was carried with runoff to the lake. From a basin where ditching and clear-cutting occurred, water, P, silica (SiO₂), N, and sodium were lost at higher rates than from five other basins. Most of the N in bulk atmospheric samples (23%) and tributary waters (56%) was dissolved organic N, a form of N not often measured.     

Seasonal Variation of Nutrient Limitation in Western Lake Erie

Monthly nutrient enrichment experiments were performed from April through October, 1983, to identify growth limiting nutrients of natural phytoplankton assemblages collected from a station in the Pigeon Bay waters off the north shore of Lake Erie's western basin. Data from these experiments suggest that silica was the major limiting nutrient in April, phosphorus was the major limiting nutrient in May-September, and trace metals were limiting in October. These results show that nutrient enrichment experiments should be performed often enough to account for seasonal changes in the physicochemical environment as well as seasonal succession in phytoplankton. The conclusion that phosphorus was the major limiting nutrient in summer is different from the conclusions of the late 1960s and early 1970s that nitrogen was the major limiting nutrient in the summer in the western basin and suggests a possible shift from nitrogen to phosphorus limitation. Pigeon Bay nutrient trend data show that summer nitrate:soluble reactive phosphorus ratios have increased from approximately 4 in the late 1960s to over 40 in the late 1970s and early 1980s, corroborating that a shift has probably occurred. This shift in nutrient limitation is probably due to reduced bioavailable phosphorus loadings and increased nitrate loadings to the western basin. The southern portion of the western basin is physicochemically different from the northern portion and may not have responded in the same manner.     

Silica Regeneration Processes in Nearshore Southern Lake Michigan

The seasonal depletion of dissolved silica to levels that limit diatom production is particularly critical in Lake Michigan's nearshore zone where diatom biomass is greatest, and where silica regeneration from sediments is not well-understood. In our study, intact, medium-fine sand cores, collected from an 11 m deep site in nearshore Lake Michigan during July-August 1980, released soluble reactive silica (SRS) at a mean rate of 2,707 ± 122 (SE) μg Si cm^?2 yr^?1 when incubated in darkness and at 12° C. This measured SRS release was greater than a diffusive flux (270 ± 49 (90% C.I.) μg Si cm^?2 yr^?1) estimated from SRS pore water profiles and physical sediment properties. SRS release from individual cores was not correlated with abundance of most macroinvertebrates (chironomids, pisidiid clams, or oligochaetes). However, a significant (P < 0.05) and inverse relationship between SRS release and Pontoporeia hoyi densities implied that amphipods suppressed SRS release through mixing and burial of a surficial floe layer, where most dissolution of biogenic silica occurs. Moreover, SRS release rates measured from our coarse-grained nearshore sediments were comparable to rates reported for fine-grained offshore material and further implicate dissolution of surficial biogenic silica as the source of remineralized SRS. Because nearshore areas of Lake Michigan undergo strong seasonal variations in temperature and diatom production, and because significant riverine silica inputs exist, we cannot extrapolate our results on a lakewide, or season-long basis. The data, however, strongly imply that nearshore sediments are an important participant in the Lake Michigan silica cycle.     

Nutrient management in Lake Erie: Evaluating stakeholder values,attitudes, and policy preferences

Harmful algal blooms (HABs) have impaired Lake Erie’s western basin water quality since the 1960s. Though scientists agree that eutrophication is a key driver of HABs in Lake Erie, the role of nitrogen is still the subject of debate. The problem is twofold: (1) uncertainty in the specific causes of HABs can lead to inappropriate management solutions, and (2) managing a cross-boundary watershed requires collaboration and agreement on apt solutions from multiple stakeholders as well as many U.S. states and Canadian provinces. In our study, we interviewed 29 stakeholders actively involved in western Lake Erie’s watershed. We analyzed the stakeholders’ values, attitudes, and policy preferences to understand their differences, similarities, and their effects on management decisions. We found that stakeholders agree on the urgency of the problem and the necessity for increased nutrient management in Lake Erie’s western basin. Furthermore, we found that stakeholders can be represented as distinct clusters based on their values, and these value-based clusters are associated with different policy preferences. The different opinions and preferences of these value clusters span across stakeholder sectors and may affect efforts toward policy change. Stakeholders often question the feasibility and effectiveness of existing policies and policy plans. The findings shed new light on the relationship between stakeholder type and environmental values, attitudes, and policy preferences. Collaboration on HABs in Lake Erie will require open lines of communication both to improve policy and to cultivate trust among the multiple parties in this diverse watershed.     

Nutrient Loading of Southern Lake Michigan by Dry Deposition of Atmospheric Aerosol

Aerosol samples and micrometeorological data were collected at 87°00’W 42°00’N in the mid southern Lake Michigan basin from May through September, 1977, to determine total phosphorus and nitrate-nitrite nitrogen particulate (TP + N) loadings. Hi-volume samplers with cellulose fiber filters and a meterological data collection system were operated on board the USEPA's R/V Roger R. Simons. A diabatic drag coefficient method was used to estimate aerosol deposition velocity (V_d), which had a mean value of 0.65 cm/s. TP + N concentration and V_d were determined for 42 distinct sampling periods of at least three to four hours each. A climatological data base was used to compute weighted average loading rates for TP + N. Dry deposition loading for the southern basin was found to be 0.15 to 0.18 × 10⁶ kg/year for P; N loading was 3.5 to 5.1 × 10⁶ kg/year. TP + N inputs via dry deposition account for 15% or more of all atmospheric nutrient inputs, and are significant nutrient sources to midlake waters.     

Nepheloid Layer in Lake Ontario

A study was designed to investigate the existence and extent of a nepheloid layer in Lake Ontario. A transmissometer profile of the entire water column was obtained monthly from 94 stations in Lake Ontario during the period of August to December, 1981. The profiles indicated that a turbid layer at the lake bottom existed over the entire lake at water depths greater than 60 meters. The thickness of the layer averaged 22 meters in August and September, but doubled to 45 meters in October. In addition, the average transmission was lowered from 60 percent in August and September to 45 percent in October. A significant increase in soluble reactive phosphorus (SRP) and reactive silica (RSiO₂) in the nepheloid layer was observed. Quartz and calcite were the major mineral constituents of the suspended matter present in the layer. The particles in the nepheloid layer in the western basin of Lake Ontario consisted of many diatoms and some calcite particles and clay minerals. In the central and eastern basins the number of calcite particles significantly increased. Concentrations of Pb and Zn in the suspended matter sampled from the nepheloid layer at the Niagara River mouth were similar to those found in the 1-cm surface sediment at the same station (124 μg g^?1 Pb, 318 μg g^?1 Zn,and139 μg g^?1 Pb and 356 μg g^?l Zn, respectively).     

Chloride and total phosphorus budgets for Lake Nipissing,headwater of Lake Huron,Ontario, Canada

Anthropogenic sources of total phosphorus (TP) and chloride (Cl^?) to lakes and rivers have been issues of concern for many decades in the Great Lakes Basin with northern Boreal Shield headwater tributaries less well studied. In the Sturgeon River – Lake Nipissing – French River basin, a headwater basin of Georgian Bay, Lake Huron, water quality monitoring of major inflows to Lake Nipissing, the third largest inland lake located entirely within Ontario, is only available from the mid-1960s to the 1990s. During the period of 2015–2018, we conducted monthly water quality surveys of major and minor inflows for two water years and have generated the first chloride (Cl^?) and total phosphorus (TP) elemental budgets for the lake. Review of available long-term concentration data indicate decreasing TP concentrations by decade in major inflows, but select inflows continue to exhibit concentrations above provincial objectives, including inflows from agricultural areas that are no longer part of provincial monitoring programs. Some inflows also show high average Cl^? concentrations with potential influences (e.g., road salt, agricultural activities) to stream water quality throughout the year. Water and elemental budgets indicate that while specific runoff (l/s/km²) is quite similar across contributing catchments, yields of Cl^? and TP (kg/ha/yr) are disproportionately higher in catchments with urban and agricultural activities. While uncertainties in the water balance and elemental yields remain, this first effort to quantify annual elemental budgets of Lake Nipissing highlights the need to develop community-based, spatially distributed water quality surveying for long-term ecosystem monitoring and future planning.     

上海市淀山湖水域春夏季水质特征及环境容量分析

根据2011-2015年间春夏季对上海市淀山湖水域中pH、水温、溶解氧、总氨氮、亚硝酸盐氮、硝酸盐氮、总氮及总磷等环境因子的调查结果,分析水体中氮磷变化特征及营养盐限制状态,采用综合营养状态指数法对水体富营养化现状进行评价,利用数学模型估算淀山湖主要特征污染物的环境容量。结果表明:调查期间TN、TP春季与夏季存在显著差异,除2014年TP季节规律不明显外,春季TN、TP含量显著高于同年夏季;淀山湖三态氮含量排序,NO_3~-—N含量最高,TAN其次,NO-2—N最低,淀山湖水域三态氮基本达到热力学平衡;氮磷Pearson相关性分析表明湖库整体呈氧化性环境,氮磷补给具有异源性。根据水中营养物限制性分类标准分析得出,大多数情况下淀山湖水域氮磷比例合理,少数情况下营养盐限制状态以磷限制为主,氮磷限制状态交替存在的情况,可能会对浮游植物生长和群落演替产生影响。综合营养状态指数法计算结果表明调查期间淀山湖均处于富营养化状态,其中2014年之前基本处于中度富营养化状态,2014年以后处于轻度富营养化状态。环境容量分析表明除2014年夏季CODMn外,调查期间淀山湖水体CODMn、TN及TP的污染负荷现状均大于同期保护目标要求的水环境容量,为达到水环境保护的目标,需要进行不同程度的削减。     

Lake trout in northern Lake Huron spawn on submerged drumlins

Recent observations of spawning lake trout Salvelinus namaycush near Drummond Island in northern Lake Huron indicate that lake trout use drumlins, landforms created in subglacial environments by the action of ice sheets, as a primary spawning habitat. From these observations, we generated a hypothesis that may in part explain locations chosen by lake trout for spawning. Most salmonines spawn in streams where they rely on streamflows to sort and clean sediments to create good spawning habitat. Flows sufficient to sort larger sediment sizes are generally lacking in lakes, but some glacial bedforms contain large pockets of sorted sediments that can provide the interstitial spaces necessary for lake trout egg incubation, particularly if these bedforms are situated such that lake currents can penetrate these sediments. We hypothesize that sediment inclusions from glacial scavenging and sediment sorting that occurred during the creation of bedforms such as drumlins, end moraines, and eskers create suitable conditions for lake trout egg incubation, particularly where these bedforms interact with lake currents to remove fine sediments. Further, these bedforms may provide high-quality lake trout spawning habitat at many locations in the Great Lakes and may be especially important along the southern edge of the range of the species. A better understanding of the role of glacially-derived bedforms in the creation of lake trout spawning habitat may help develop powerful predictors of lake trout spawning locations, provide insight into the evolution of unique spawning behaviors by lake trout, and aid in lake trout restoration in the Great Lakes.     

Nutrient footprints on the Toronto-Mississauga waterfront of Lake Ontario

The extent of nutrient enrichment over the urbanized shoreline of western Lake Ontario bordering the Cities of Toronto and Mississauga was investigated in 2018. Concentrations of total phosphorus were higher and more wide-ranging compared with nearshore reference areas in eastern Lake Ontario. Area-weighted chlorophyll a was higher over the shoreline from the mouth of the Credit River to Humber Bay west of Toronto Harbour compared with that to the east from the Toronto Beaches to the Rouge River, a pattern also seen broadly in concentrations of phosphorus fractions and nitrates. Concentrations of phosphate measured by mass spectrometry, and using an internal standard to minimize sampling losses, were higher in Toronto Inner Harbour, near wastewater outfalls, river mouths, and to varying extent over the mixing areas of these discharges. Chlorophyll a suggested broadly oligotrophic to oligo-mesotrophic conditions despite areas with nutrient fractions suggestive of more productive conditions. Toronto Inner Harbour was mesotrophic, with wide-ranging levels of phosphorus fractions strongly influenced by the variable loading from the Don River into the harbour. Phosphorus fractions in the harbour quays which also receive CSOs discharges were elevated at times. Blooms or scums of planktonic algae were not observed. Elevated concentrations of phosphate were found at shallow depths suitable for growth of the green algae Cladophora. As these urban areas continue to grow, the potential for additional nutrient loading to exacerbate growth of algae should be watched given the present conditions that indicate variable but pervasive nutrient enrichment.     

Nutrient Changes in Saginaw Bay,Lake Huron,After the Establishment of the Zebra Mussel (Dreissena polymorpha)

Concentrations of particulate and dissolved nutrients in Saginaw Bay, Lake Huron, were examined relative to zebra mussel colonization which occurred summer 1991. The magnitude and spatial pattern of changes indicate that mussels had a significant impact on nutrients in Saginaw Bay. Annual means for total suspended solids, particulate organic carbon, particulate phosphorus, and particulate silica in the inner bay were significantly lower in 1992 and 1993 (post-zebra mussel) than in 1991 (pre-zebra mussel). Annual means decreased from 11.5 mg L^−l, 1.45 mg C L^−l (121 μM), 20.4 μg P L⁻¹(0.66 μM), and 1.52 mg SiO₂ L⁻¹ (24 μM) respectively in 1991 to 4.4 mg L⁻¹, 0.79 mg C L⁻¹(66 μM), 11.2 μg P L^−l (0.36 μM), and 0.77 mg SiO₂ L⁻¹(12 μM) in 1993. In contrast, there were no significant differences among years for these parameters at control stations, which were located in the outer bay and had no known populations of mussels. Annual means for nitrate, ammonium, and silica were significantly higher in the inner bay in 1992 than in 1991, but not significantly different in 1993. Means increased from 0.39 mg N L⁻¹, 21.0 μg N L^−], and 1.11 mg SiO₂ L⁻¹ respectively in 1991 to 0.47 mg N L⁻¹, 30.9 μg N L⁻¹, and 1.71 mg SiO₂ L⁻¹ in 1992. No significant differences were observed for these parameters in the control group. Differences between 1992 and 1993 may reflect differences in the amount of runoff and circulation between Saginaw Bay and Lake Huron.A phosphorus budget indicated that zebra mussels were a significant sink for phosphorus. Mussels from the inner bay accumulated 108, 682, and 52 t respectively in 1991, 1992, and 1993. Comparatively, the annual pool of phosphorus in the water column of the inner bay decreased from a pre-zebra mussel (1979–1980) average of 712 t to 421 and 382 t in 1992 and 1993 respectively.     

两湖（库）的氮循环及富营养化

通过检测资料的分析，指出两湖（库）出现富营养化现象及氮是起湖（库）水体富营养化的控制因子，季节性的藻类繁殖是促成两湖（库）水质污染突变恶化的主要原因。     

In situ, high-resolution time series of dissolved phosphate in Green Bay,Lake Michigan

In nearly every instance in which the environment has been sampled on a higher resolution in time or space, fundamental processes have come to light that were previously undetected or unobserved. In this study, an autonomous dissolved phosphate sensor was deployed at the Entrance Light station in lower Green Bay, Lake Michigan in 2012 and 2013. Hourly phosphate sensor measurements were compared with other real time sensor data to gain insights into the processes occurring at this site. Results showed that the water column at this location undergoes repeated stratification and turnover during the course of the summer. Often, the stratification results from intrusions of cold hypolimnetic bottom water from the north, while turnover is associated with significant northerly and/or easterly wind events. It was observed that, during calm periods, dissolved phosphate concentrations increased at a rate that was stoichiometrically consistent with the consumption of dissolved oxygen during the remineralization of organic matter; specifically, areal oxygen consumption rates ranged from 3.2 to 43?mmol?m^?2?d^?1 and oxygen to phosphate ratios ranged from 120 to 210. At other times, the inverse relationship between dissolved oxygen and dissolved phosphate was not stoichiometrically linked; during these times, areal oxygen depletion rates ranged from 51 to 240?mmol?m^?2?d^?1 and oxygen to phosphate ratios ranged from 260 to 2300. Future strategic deployment of multiple in situ dissolved phosphate and other nutrient sensors will enhance the understanding of nutrient cycling in this important aquatic system.     

瓦埠湖沉积物中氮与磷赋存形态分析

利用连续提取法研究了瓦埠湖沉积物中磷的赋存形态和氮的组成特征。结果显示:瓦埠湖沉积物中总磷含量为293～874 mg/kg,平均为470 mg/kg,总磷主要由无机磷组成;在沉积物的提取磷中铁结合态磷为主要赋存形态;铁磷、钙磷、铝磷的相对百分含量指示出该湖泊的磷源自污染较少的区域。瓦埠湖沉积物中总氮的含量为208～464 mg/kg,平均为265 mg/kg,氮的主要组成为有机氮,有机氮、有机指数等显示该湖泊属于清洁类型,未遭受污染。瓦埠湖流域的社会与自然状况的调查结果、湖泊底泥的有机碳氮比等均表明湖泊营养物质来自面源,各指标含量在空间分布的特点与相关性的差异,说明了物质沉积存在重力分离与水动力作用的过程。     

Status of a stocked Atlantic salmon population in Lake Huron

In 2011, the Michigan Department of Natural Resources (MDNR) expanded stocking of Atlantic salmon (Salmo salar) in Lake Huron to enhance fishing opportunities following Chinook salmon (Oncorhynchus tshawytscha) abundance declines. Currently, little is known about the population produced from this stocking. We fit an assessment model to harvest data from Michigan jurisdictional waters to estimate Atlantic salmon population dynamics and abundance. Because of potential biases in MDNR creel survey harvest estimates, a survey was e-mailed to online purchasers of a 2019 Michigan fishing license asking about where and when Atlantic salmon were harvested to correct creel harvest estimates. Anglers were also asked about catch-and-release angling and tested on their ability to identify Lake Huron salmonids. Creel harvest estimates overlapped spatially and temporally with 42% of survey reported Atlantic salmon harvest. After correcting creel harvest estimates, total abundance of Atlantic salmon in 2019 was estimated at approximately 392,000 fish with a peak abundance of approximately 406,000 fish. Anglers released 27% of caught fish and correctly identified Atlantic salmon 28% of the time. To assess the occurrence of food resource competition, differences in condition (i.e., expected weight at length) were evaluated. Condition was higher in later years than in earlier years despite abundance increases. Our results suggested that past stocking established a population of approximately 400,000 Atlantic salmon with evidence suggesting that fish were finding sufficient food resources. Future Atlantic salmon management efforts may be improved by quantifying post-stocking survival rates and other sources (e.g., charter, Canadian) of harvest.     

In Situ Determination of the Annual Thermal Habitat Use by Lake Trout (Salvelinus namaycush) in Lake Huron

Records of the temperatures occupied by 33 lake trout (Salvelinus namaycush) at large in Lake Huron were obtained for up to 14 months per fish, at 75-minute intervals, from surgically implanted archival temperature tags. The dataset covered nearly three years, from October 1998 to June 2001, and included 160,000 observations. The objectives of the tagging were to obtain temperature data to refine bioenergetics models of sea lamprey (Petromyzon marinus) predation on lake trout, and compare the temperatures occupied by strains of lake trout stocked in Lake Huron. The seasonal, thermal-use profiles of lake trout followed the general warming and cooling pattern of Lake Huron. During periods when the zone of surface water mixing extended below the depth range occupied by lake trout, variability among individual fish and strains was low and followed surface temperature. However, during the period of summer stratification, the average temperatures occupied varied substantially among individual fish and strains. Strains originating from the upper Great Lakes (Lake Superior and Lewis Lake, WY) occupied similar temperatures. Between June and mid August, upper Great Lakes lake trout typically occupied water several degrees warmer than that occupied by lake trout of Finger Lakes, New York origin. Most of the lake trout occupied summer temperatures lower than the preferred temperatures suggested by laboratory studies. In October, all strains occupied water as warm or warmer than that occupied in summer, which may partially explain the higher lethality of sea lamprey attacks during October.     

Effects of flooding regime upon the decomposition of Eichhornia azurea (Sw.) Kunth measured on a tropical,flow‐regulated floodplain (Paraná River,Brazil)

Decomposition of macrophytes is an important process in river‐floodplain systems, especially in the Upper Paraná River floodplain, given that this ecosystem receives high inputs of detritus from this vegetation. Release of nutrients by decomposition is essential in this floodplain because it is located downstream from a reservoir chain where nutrients are being trapped. Water level fluctuations are considered one of the most important aspects that affect macrophyte decomposition. Anthropogenic alterations, such as the control of flooding regimes, observed in this floodplain, could change the dynamic of this process. To evaluate the influence of the hydrological cycle upon the decomposition of Eichhornia azurea (an aquatic macrophyte that has high biomass values in this ecosystem), litter bags with senescent leaves and petioles of this plant were submitted to four different flooding treatments, which differed in time of flooding and exposure to dry conditions. The decomposition rates and the detritus chemical composition (nitrogen and phosphorus concentrations) were measured over 113 days. There were significant effects of the flooding treatments and time upon all parameters. The materials that decomposed with flood conditions showed the greatest decomposition rates. The quality of the detritus seems to be highly linked with the flooding regime, showing highest releases of phosphorus in the submerged treatments. It was shown that floods, even the short duration ones, increase the decomposition velocity and the nutrient cycling relative to dry conditions. Thus, investigations that assess the nutrient budgets on the Upper Paraná River floodplain and the role of nutrients in its productivity should consider the detritus compartment and the effects of flood regimes upon its dynamics. Copyright © 2006 John Wiley & Sons, Ltd.     

White and longnose sucker diet composition in Lake Michigan and Saginaw Bay,Lake Huron

Sucker species (Catostomidae) are a common benthic invertivore in North American waterbodies yet are understudied in the Laurentian Great Lakes. Despite large biomass and potential competition with more economically valuable fish species, the diets of Great Lakes suckers are poorly described. We explored the gut contents of adult white suckers (Catostomus commersonii) in Lake Michigan and Saginaw Bay, Lake Huron, and longnose suckers (Catostomus catostomus) in Lake Michigan. Chironomidae was a primary prey for white suckers in both lakes, along with Amphipoda in Saginaw Bay. For longnose suckers in Lake Michigan, gut contents were dominated by Amphipoda and Isopoda. Relative to other benthic invertebrate taxa, white suckers positively selected Chironomidae as preferred prey (indexed via Chesson’s alpha). Moreover, the average length of consumed Chironomidae and Amphipoda increased with white sucker length, suggesting preferential size selection of prey as suckers grow. Although dreissenid mussels are overwhelmingly abundant in the benthos of Lake Michigan and Lake Huron, relatively few dreissenid mussels were consumed by either sucker species. Thus, suckers are unlikely to contribute to the control of these invasive invertebrates.     

衡水湖水体氮磷变化规律及富营养化现状分析

衡水湖是华北平原具有独特自然景观的国家级湿地和鸟类自然保护区,富营养化是其面临的一个主要问题。通过对衡水湖水体的主要营养物质氮、磷的污染特征、来源、成因分析以及对水体的富营养化现状进行评价,表明衡水湖的水质状况近年来得到了一定程度的改善,但氮、磷的污染仍相当严重,污染程度冬春季较轻,夏秋季节较重,富营养化程度从中营养到轻度、中度富营养。     

Seventy years of food-web change in South Bay,Lake Huron

Though aquatic ecosystems (and the Laurentian Great Lakes in particular) have faced many stressors over the past century, including fisheries collapses and species invasions, rarely are data available to evaluate the long-term impacts of these stressors on food web structure. Stable isotopes of fish scales from the 1940s to the 2010s in South Bay, Lake Huron were used to quantify trophic position and resource utilization for fishes from offshore (alewife, cisco, lake trout, lake whitefish, rainbow smelt) and nearshore (rock bass, smallmouth bass, white sucker, yellow perch) habitats, providing one of the longest continuous characterizations of food webs in the Laurentian Great Lakes. Mean δ¹⁵N and δ¹³C values for each species were compared across twenty-year time periods. Using directional statistics, no significant community-wide changes were detected between time periods from 1947 to 1999. In contrast, a significant change was detected between 1980-1999 and 2000–2017, with all species showing increased reliance on nearshore resources. The increase in nearshore resource reliance for lake whitefish between these time periods was the greatest in magnitude compared with any other species between any two adjacent time periods. Besides lake whitefish, the increased reliance on nearshore resources was more pronounced for nearshore compared to offshore species. The timing of these shifts coincided with the invasion of dreissenid mussels and round goby, and declines in offshore productivity and prey densities. These results show the unprecedented magnitude of recent food-web change in Lake Huron after 50 years of relative stability.