Subsidy or subtraction: how do terrestrial inputs influence consumer production in lakes?

Cross-ecosystem fluxes are ubiquitous in food webs and are generally thought of as subsidies to consumer populations.  Yet external or allochthonous inputs may in fact have complex and habitat-specific effects on recipient ecosystems.  In lakes, terrestrial inputs of organic carbon contribute to basal resource availability, but can also reduce resource availability via shading effects on phytoplankton and periphyton.  Terrestrial inputs might therefore either subsidise or subtract from consumer production.  We developed and parameterised a simple model to explore this idea.  The model estimates basal resource supply and consumer production given lake-level characteristics including total phosphorus (TP) and dissolved organic carbon (DOC) concentration, and consumer-level characteristics including resource preferences and growth efficiencies.  Terrestrial inputs diminished primary production and total basal resource supply at the whole-lake level, except in ultra-oligotrophic systems.  However, this system-level generalisation masked complex habitat-specific effects. In the pelagic zone, dissolved and particulate terrestrial carbon inputs were available to zooplankton via several food web pathways.  Consequently, zooplankton production usually increased with terrestrial inputs, even as total whole-lake resource availability decreased.  In contrast, in the benthic zone the dominant, dissolved portion of the terrestrial carbon load had predominantly negative effects on resource availability via shading of periphyton.  Consequently, terrestrial inputs always decreased zoobenthic production except under extreme and unrealistic parameterisations of the model.  Appreciating the complex and habitat-specific effects of allochthonous inputs may be essential for resolving the effects of cross-habitat fluxes on consumers in lakes and other food webs

2010 Status of the Lake Ontario Lower Trophic Levels

This report presents data on the status of lower trophic level components of the Lake Ontario ecosystem (zooplankton, phytoplankton, nutrients) in 2010 and compares the 2010 data with available time series. Lower trophic levels are indicators of ecosystem health [as identified by the Lake Ontario Pelagic Community Health Indicator Committee (EPA 1993) and presented in the biennial State of the Lake Ecosystem Conference (SOLEC) reports] and determine the lake’s ability to support the prey fish upon which both wild and stocked salmonids depend. Understanding the production potential of lower trophic levels is also integral to ecosystem-based management. Continued evaluation of lower trophic levels is particularly important for fisheries management, as the observed declines in alewife and Chinook salmon in Lake Huron in 2003 may have been partly the result of changes in lower trophic levels (Barbiero et al. 2009)

Do Daphnia use metalimnetic organic matter in a north temperate lake? An analysis of vertical migration

Diel vertical migration of zooplankton is influenced by a variety of factors including predation, food, and temperature. Research has recently shifted from a focus on factors influencing migration to how migration affects nutrient cycling and habitat coupling. Here we evaluate the potential for Daphnia migrations to incorporate metalimnetic productivity in a well-studied northern Wisconsin lake. We use prior studies conducted between 1985 and 1990 and current diel migration data (2008) to compare day and night Daphnia vertical distributions with the depth of the metalimnion (between the thermocline and 1% light depth). Daphnia migrate from a daytime mean residence depth of between about 1.7 and 2.5 m to a nighttime mean residence depth of between 0 and 2.0 m. These migrations are consistent between the prior period and current measurements. Daytime residence depths of Daphnia are rarely deep enough to reach the metalimnion; hence, metalimnetic primary production is unlikely to be an important resource for Daphnia in this system

25th annual computational neuroscience meeting: CNS-2016

The same neuron may play different functional roles in the neural circuits to which it belongs. For example, neurons in the Tritonia pedal ganglia may participate in variable phases of the swim motor rhythms [1]. While such neuronal functional variability is likely to play a major role the delivery of the functionality of neural systems, it is difficult to study it in most nervous systems. We work on the pyloric rhythm network of the crustacean stomatogastric ganglion (STG) [2]. Typically network models of the STG treat neurons of the same functional type as a single model neuron (e.g. PD neurons), assuming the same conductance parameters for these neurons and implying their synchronous firing [3, 4]. However, simultaneous recording of PD neurons shows differences between the timings of spikes of these neurons. This may indicate functional variability of these neurons. Here we modelled separately the two PD neurons of the STG in a multi-neuron model of the pyloric network. Our neuron models comply with known correlations between conductance parameters of ionic currents. Our results reproduce the experimental finding of increasing spike time distance between spikes originating from the two model PD neurons during their synchronised burst phase. The PD neuron with the larger calcium conductance generates its spikes before the other PD neuron. Larger potassium conductance values in the follower neuron imply longer delays between spikes, see Fig. 17.Neuromodulators change the conductance parameters of neurons and maintain the ratios of these parameters [5]. Our results show that such changes may shift the individual contribution of two PD neurons to the PD-phase of the pyloric rhythm altering their functionality within this rhythm. Our work paves the way towards an accessible experimental and computational framework for the analysis of the mechanisms and impact of functional variability of neurons within the neural circuits to which they belong

Dissolved organic carbon concentration controls benthic primary production: Results from in situ chambers in north-temperate lakes

Abstract We evaluated several potential drivers of primary production by benthic algae (periphyton) in north-temperate lakes. We used continuous dissolved oxygen measurements from in situ benthic chambers to quantify primary production by periphyton at multiple depths across 11 lakes encompassing a broad range of dissolved organic carbon (DOC) and total phosphorous (TP) concentrations. Light-use efficiency (primary production per unit incident light) was inversely related to average light availability (% of surface light) in 7 of the 11 study lakes, indicating that benthic algal assemblages exhibit photoadaptation, likely through physiological or compositional changes. DOC alone explained 86% of the variability in log-transformed whole-lake benthic production rates. TP was not an important driver of benthic production via its effects on nutrient and light availability. This result is contrary to studies in other systems, but may be common in relatively pristine north-temperate lakes. Our simple empirical model may allow for the prediction of whole-lake benthic primary production from easily obtained measurements of DOC concentration

Cooperative science to inform Lake Ontario management: Research from the 2013 Lake Ontario CSMI program

Since the mid-1970s, successful Lake Ontario management actions including nutrient load and pollution reductions, habitat restoration, and fish stocking have improved Lake Ontario. However, several new obstacles to maintenance and restoration have emerged. This special issue presents management-relevant research from multiple agency surveys in 2011 and 2012 and the 2013 Cooperative Science and Monitoring Initiative (CSMI), that span diverse lake habitats, species, and trophic levels. This research focused on themes of nutrient loading and fate; vertical dynamics of primary and secondary production; fish abundance and behavior; and food web structure. Together these papers identify the status of many of the key drivers of the Lake Ontario ecosystem and contribute to addressing lake-scale questions and management information needs in Lake Ontario and the other Great Lakes and connecting water bodies

Stable isotope turnover and half-life in animal tissues: a literature synthesis.

Stable isotopes of carbon, nitrogen, and sulfur are used as ecological tracers for a variety of applications, such as studies of animal migrations, energy sources, and food web pathways. Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. There have been a large number of experimental isotopic diet shift studies aimed at quantifying animal tissue isotopic turnover rate λ (%·day(-1), often expressed as isotopic half-life, ln(2)/λ, days). Yet no studies have evaluated or summarized the many individual half-life estimates in an effort to both seek broad-scale patterns and characterize the degree of variability. Here, we collect previously published half-life estimates, examine how half-life is related to body size, and test for tissue- and taxa-varying allometric relationships. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs. Half-life was longest in ecotherms, followed by mammals, and finally birds. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept; n = 261, p<0.0001, r2 = 0.63. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. While isotopic half-life can be approximated using simple allometric relationships for some taxa and tissue types, there is also a high degree of unexplained variation in our models. Our study highlights several strong and general patterns, though accurate prediction of isotopic half-life from readily available variables such as animal body mass remains elusive

Analysis of Lake Ontario Lower Aquatic food web Assessment (LOLA 2003 and 2008) within the context of long-term ecological change

Lake Ontario is the 13th largest lake in the world with a surface area of 18,500 km² (Reynolds et al. 2000), has a population in the watershed of over 8 million, and provides a range of ecosystem services to the people in the watershed (freshwater for various uses, shipping, fisheries, and recreation). Currently, extensive surveys for each Great Lake occur on a rotating five-year schedule. This report presents the status of Lake Ontario’s lower trophic levels in 2008 and a detailed comparison with similarly collected in 2003 and with data collected by the collaborating agencies and Cornell University and discuss observed changes in relation to changes in nutrient concentration and food web configuration in Lake Ontario. There has been a spatial restructuring of the Lake Ontario offshore ecosystem through the increase in the deep chlorophyll layer and associated zooplankton. This has resulted in a Lake Ontario that in 2008 is more similar to Lakes Superior, Huron and Michigan than to the Lake Ontario of the 1990s. Major findings are Nutrients: Spring offshore total phosphorus and soluble reactive phosphorus increased from 2003 to 2008, but summer levels did not. Lake-wide average total phosphorus levels remained at or below the target level of 10 µg/L in all three seasons of 2008. Lake-wide nutrient concentrations have declined since the 1960s. However, phosphorus concentrations have been stable (~7-10 µg/L) since the mid-1990s. Spring silica was similar in 2003 and 2008 and was depleted by the summer in both years. This indicates continued spring diatom production in Lake Ontario. Phytoplankton: Summer epilimnetic chlorophyll-a increased by a factor of 2, the proportion of autotrophic algae increased, and summer water clarity declined from 2003 to 2008. Summer chlorophyll-a levels in 2008 were similar to the concentrations in the 1981-1995 time period. However, the trend towards mesotrophy in the summer of 2008 may be limited to that year as it was not followed by increased values in 2009 to 2011. Most of the chlorophyll in the water column was located in a deep chlorophyll layer in the thermocline. Zooplankton: Offshore epilimnetic zooplankton density and biomass declined from 2003 to 2008 by a factor of 5 to 12 in the summer and by a factor of 1.5 to 2.6 in the fall. This is consistent with long-term trends of declining epilimnetic zooplankton abundance including a larger decline in 2004-2005 coincident with an increase in the predatory Bythotrephes. Whole water column zooplankton density also declined from 2003 to 2008 in the summer and fall, but zooplankton biomass only declined in the fall. Large changes in whole water column zooplankton community composition occurred between 2003 and 2008 from a cyclopoid/bosminid dominated system in 2003 to a calanoid dominated system in 2008. Mysids, Diporeia and Mussels Mysid densities were similar in 2003 and 2008 indicating continued high biomass of mysids in Lake Ontario. In July of 2008, the biomass of Mysis diluviana was 17% of the crustacean zooplankton biomass in the offshore of Lake Ontario (depth >30m). Mysid densities appear stable in Lake Ontario. The native benthic amphipod Diporeia declined further in 2008 and is almost extirpated from Lake Ontario. Quagga mussels are very abundant as deep as 90 m, but populations in shallow water declined from 2003 to 2008. Few zebra mussels were present in either 2003 or 2008.Report for the GLRI funded project Improving Lake Ontario Environmental Management Decisions (Grant ID # 97220700-0