It takes an army to do this….!
Shallow Lakes Are Special!
“Special”? Not everybody thinks so…
• Shallow lakes have changed profoundly!
Many shallow lakes are in lousy shape!
“In practice, very few lakes that have collapsed to the turbid state have been restored successfully to the clearwater state (Osgood 2000; NRC 1992)” (cited by Carpenter et al. 2001, p. 772)
Talking Points: Q 1: What are the implications of shallow depth? Q 2: What are some major factors that influence shallow lake conditions? Q 3: Why is it so hard to manage shallow lakes?
Q 1: What are the implications of “shallowness”? 1. Light levels are often much higher at the sediments
2. Wave energy reaches sediments when windy 3. Water column is frequently mixed – “polymixis” is the rule
4. Fish often play important roles in establishing ecological conditions
• Fish have stronger influences in shallow lakes compared to deeper lakes-why? (Reviewed by Jeppesen et al. 1997)
a) Fish abundance is much higher in shallow lakes (deep lakes 10-100 kg/ha, shallow lakes 350-1000 kg/ha). Why? Shallow lakes: a) are more productive b) fish can utilize the entire lake bottom
b) Fish predation on invertebrates is more intense in shallow lakes. Why? - higher fish densities - invertebrates have no deep water refuge
c) Shallow lake fish communities have a lower proportion of predatory fish like pike or bass. Why? - predatory fish don’t handle low DO levels – especially during long Midwestern winters
so? - favors high densities of carp, bullheads, and other bottomfeeding fish
d) Fish are more likely to eat and resuspend lake sediments in shallow lakes. This means- decreased water clarity and pumping nutrients into the water column…where there is plenty of light for algae
Q. 2: What are some major influences on shallow lake conditions?
(Again) Watersheds vs. in-lake features: which has more influence on among-lake patterns in aquatic invertebrates? Lake-level variables: Surface Area Depth Nutrients in water column Phytoplankton biomass Submerged macrophyte extent Fish communities Carp – P/A Bullheads – P/A Planktivores (PL) - mass Benthivores (BE) – mass
ECS levels – 3 Provinces, 5 Sections NLCD – land-use categories (US11-US45) Up-stream watershed area (UWA) “Influence ratios” – UWA:Lake surface area % disturbed uplands in watershed % agr. (row crops) in uplands
Response: -Zooplankton species -Macroinvertebrate species -Abundance, taxon richness, aggregate variables
Multi-scale shallow lake analysis
Used 2-step approach to predicting aquatic invertebrate community pattern; we modeled 1. Invertebrate responses to: in-lake (dynamic) variables 2. Predicted lake effects (from #1) relationship to: watershedscale (static) variables
In-lake factors (http://onlinecmag.com)
Watershed - scale effects – how much? (Wikipedia.org)
Not very much!
AGR = Agriculture; GRA = Grassland; URB = Impervious surfaces and Residential
Best Models to Predict Chla?
Best: “Sumfish + TP” Better: “Sumfish +TP + Year” Good: “Sumfish + TP” + S*T”
AICc 71.1 71.7 71.9
“Chla concentration was best explained by regional and temporal factors and there was no significant relationship between agriculture and Chla concentration.” (Bayley et al. 2013)
Q. 3: Why Is It So Hard To Rehabilitate Shallow Lakes?
(20-40%?) (Journal of Applied Ecology 2007: 1095-1105)
Resilience and historical legacies are partly to blame
NORTHERN MINNESOTA WETLANDS
LAKE AGASSIZ PLAINS
NORTHERN LAKES AND FORESTS
NORTH CENTRAL HARDWOOD FORESTS
NORTHERN GLACIATED PLAINS
DRIFTLESS AREA WESTERN CORN BELT PLAINS
“…resilience can delay… establishment of clear water even when external nutrient loading is…low…” (Jeppesen et al. 2012, p. 413)
Q: what causes this in shallow lakes? 1. Chemical factors: -P (N?) from lake sediments 2. Biological factors: -Fish communities -Slow responses by macrophytes 3. Hydrological factors (probably have all of these…)
About 10 -15 yrs (or <)? (Hobbs et al. 2012)
Legacies define modern shallow lake conditions!
More broadly – “Paleolimn” on 11 MN lakes • Methods can classify clear, turbid, transitional lakes • Records suggest often a single transition sometime after 1920 in turbid lakes • Little evidence that TP is a single inducer • Some clear-state lakes stable since before 1700!
If we had a decision tool…what then? Suppose you’re a lake manager….
-If you could manage only 1 of these, should you treat “green” P Steady State vs. L or “blue” lake?
Unstable 0 0
More broadly… N < N1 Protect (highly resilient clear lake)
Lake conditions N1 < N < N 2 Active Mgmt
N > N2
High Cost (highly resilient turbid lake)
N < N1
N > N2
N1 < N < N 2
(highly resilient clear lake)
A < Acrit Vulnerable Clear Lake • • •
(highly resilient turbid lake)
A > Acrit Opportunistic Turbid Lake
Distance to thresholds Transition probabilities Value to wildlife/humans
Management Decisions • • • •
Apply fish toxicants? Drawdown water levels? Stock piscivorous fish? More data??
Final Reflections -what do we do now? • Incorporate implications of resilience in expectations & management • Manage surface connectivity • Respond with both watershed- and in-lake measures • Use “tiered approaches” to protect hi-quality sites! Don’t give up hope – lakes will surprise us!
Summer TP (measured and inferred)
2015 Why ? -reduced feedlot runoff -retire erodible lands -created wetlands