We examined stressors that have led to profound ecological changes in the Lake Ontario ecosystem and its fish community since 1970. The most notable changes have been reductions in phosphorus loading, invasion by Dreissena spp., fisheries management through stocking of exotic salmonids and control of sea lamprey (Petromyzon marinus), and fish harvest by anglers and double-crested cormorants (Phalacrocorax auritus). The response to these stressors has led to (i) declines in both algal photosynthesis and epilimnetic zooplankton production, (ii) decreases in alewife (Alosa pseudoharengus) abundance, (iii) declines in native Diporeia and lake whitefish (Coregonus clupeaformis), (iv) behavioral shifts in alewife spatial distribution benefitting native lake trout (Salvelinus namaycush), threespine stickleback (Gasterosteus aculeatus), and emerald shiner (Notropis atherinoides) populations, (v) dramatic increases in water clarity, (vi) predation impacts by cormorants on select fish species, and (vii) lake trout recruitment bottlenecks associated with alewife-induced thiamine deficiency. We expect stressor responses associated with anthropogenic forces like exotic species invasions and global climate warming to continue to impact the Lake Ontario ecosystem in the future and recommend continuous long-term ecological studies to enhance scientific understanding and management of this important resource. 490Résumé : On trouvera ici un examen des facteurs de stress qui ont modifié profondément l'écosystème du lac Ontario et sa communauté de poissons depuis 1970. Les changements les plus importants ont été la réduction de l'apport de phosphore, l'invasion des Dreissena spp., la gestion de la pêche, notamment l'empoissonnement de salmonidés exotiques et le contrôle de la grande lamproie marine (Petromyzon marinus), ainsi que la récolte des poissons par les pêcheurs sportifs et les cormorans à aigrette (Phalacrocorax auritus). La réaction à ces facteurs a eu pour conséquen-ces: (i) le déclin de la photosynthèse des algues et de la production du zooplancton épilimnétique, (ii) la diminution de l'abondance du gaspareau (Alosa pseudoharengus), (iii) la réduction des Diporeia indigènes et des grands corégonesCan.
Results from long-term ecological monitoring studies on Lake Michigan (1983–92) and Lake Ontario (1981–92) were compared with regard to changes in phosphorus loads. In Lake Ontario, total phosphorus (TP) loads decreased from 14 000 t∙yr−1 in 1970 to 7500 t∙yr−1 in 1981, and correspondingly, midlake TP concentration decreased from 25 to 16 μg∙L−1 From 1981 to 1991 TP loads remained around 7500 t∙yr−1; however, TP concentration continued to decline from 16 to 10 μg∙L−1. Similarly, mean summer particulate organic carbon (POC), chlorophyll a (CHLa), and nitrate utilization rate decreased by approximately 40, 20, and 50%, respectively. Conversely, silica utilization rates increased markedly after 1983. In Lake Michigan, TP loads also decreased by around 50% from 1974 to 1990 (2000 t∙yr−1); however, TP concentrations at our 100-m station in the southern basin increased during the study period from around 4 to 8 μg∙L−1 There were no distinct trends in CHLa or nutrient utilization patterns; however, POC levels decreased sharply after 1987. Overall, silica utilization rates in Lake Michigan were 50% higher than in Lake Ontario (14.7 vs. 9.6 μg∙L−1∙d−1), whereas nitrate utilization rates were only half (1.4 vs. 4.1 μg∙L−1∙d−1).
Recent environmental changes in the Bay of Quinte, Lake Ontario, have coincided with a decline in the stocks of walleye Sander vitreus. Suitable habitat supply was estimated in three sections of the bay during the summers of 1972–2001 to assess its role in the decline. An empirical model was developed to predict suitable habitat area for walleyes based on their preferences for cool water and low light intensity. The results indicated that lack of suitable light limits walleye habitat in the bay. Walleye habitat in the shallow upper bay has decreased at the rate of 34 ha/year since the invasion of dreissenid mussels in 1994, while that in the middle and lower bays has remained abundant. Walleye stocks and suitable habitat in the upper bay have both declined since the early 1990s. However, this pattern has not been consistent through time and suggests that other factors have also affected the Bay of Quinte walleye population. The analyses developed here can be used as a tool to enhance the assessment of walleye habitat dynamics in the Bay of Quinte and allow us to examine the impact of oligotrophication on the habitat of an important recreational and commercial species.
The substrate [cx-~~P]ATP was used to represent the available fraction of the dissolved phosphomonoester (PME) pool and to measure its rate of assimilation by freshwater plankton in systems of low phosphorus availability. We compared [32P]ATP uptake to inorganic 32P0,3-uptake. The upper bounds to the pool sizes of POd3-and PME were estimated with Rigler bioassays, and these values, together with the uptake constants at ambient substrate concentrations, permit comparison of the uptake of organic and inorganic P by limnetic plankton. Affinity of the plankton for both substrates was high, and the estimated flux rates from both pools were similar, demonstrating that PME contributes significantly to total P uptake. We tested the hypothesis that algae > 1 km preferentially assimilate P from PME but observed that the 0.2-1-lrn size fraction dominates uptake of P from either source at ambient concentrations. The specific flux of P from PME into the 1-12qm size fraction was often as high as that into the 0.2-l-pm fraction, but plankton > 12 pm typically incorporate ~5% of the P from either source.The availability and fate of dissolved organic phosphorus (DOP) and its relative importance to limnetic plankton have not been clearly resolved due to the lack of direct methods for its quantification.When the concentration of inorganic phosphate (POq3-) is low, a significant fraction of dissolved P may be DOP, which studies have ' Present address:
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