Water quality conditions and food web structure in Chequamegon Bay

Joel Hoffman, US Environmental Protection Agency Mid-Continental Ecology Division, Duluth, MN
Presentation at the Chequamegon Bay Research Symposium, April 14, 2015

Transcript

1. Photo image area measures 2” H x 6.93” W and can be masked by a collage strip of one, two or three images. The photo image area is located 3.19” from left and 3.81” from top of page. Each image used in collage should be reduced or cropped to a maximum of 2” high, stroked with a 1.5 pt white frame and positioned edge-to-edge with accompanying images. Joel C Hoffman*, John R Kelly, Greg S Peterson, Anne M. Cotter, Matthew A. Starry, Michael E. Sierszen Water quality conditions and food web structure in Chequamegon Bay Office of Research and Development National Health and Environmental Effects Research Lab Chequamegon Bay Symposium April 14, 2015
2. Acknowledgements 1 • J. Van Alstine, M. Pearson, T. Corry, A. Trebitz, and C. Butterworth for field assistance • A. Just, L. Seifert, and M. Knuth for laboratory assistance
3. 2 Introduction • Coastal wetlands are ‘metabolic gates’ that also provide nursery habitat for many Great Lakes fishes • Within coastal wetlands, energy and nutrient exchanges are facilitated by seiche-driven mixing of lake and river water • Are exchanged energy and nutrients supporting fish larvae production? • What is the role of anthropogenic nutrients?
4. Wetland-lake linkages • Wetlands are geochemical mixing zones • Expect large geochemical gradients –Stable isotope studies –Otolith microchemistry studies • Potentially identify which habitats larvae use • Track movements between wetlands and nearshore 3 Seiche Tributary Watershed Great Lake Coastal Wetland
5. 4 ‘Estuarine’ mixing Hoffman et al. 2010. Estuaries and Coasts.
6. Isotope mixing 5 Hoffman et al. 2010. Estuaries and Coasts. Station U C L S D 13C -32 -28 -24 -20 B. freyi Cyclopoids Daphnia spp. L. kindti S. oregonensis B. longimanus D. birgei H. gibbosus Lake Superior river Seiche Tributary Watershed Great Lake Coastal Wetland Mg (mg L-1) 2 4 6 8 10 DIC  13 C -14 -12 -10 -8 -6 -4 -2 0 1:1 river:lake 2:1 river:lake Lake Superior river
7. Growth (Weight t : Weight yolk-sac ) 0 20 40 60 whole larvae 15N, ‰ 2 4 6 8 10 12 Allouez Bay Superior Inlet Chequamegon Bay rainbow smelt (Osmerus mordax) yolk-sac late larval stage Modified from Hoffman et al. 2011
8. Study Methods • Fish larvae collections weekly from April through mid-July –x5 reps/species/size class/station –Rep = composite 2-10 fish – 15N, 13C (±0.1‰) • Surface water sampled every other week –Cl-, Mg++ –Nutrients (NH4 , NOx , TN, SRP, TP) –TSS –Chl a • Correlation, regression, multiple linear regression 7
9. 8 Water quality stations Fish larvae stations
10. 9 Water Quality Chequamegon Bay 2008 Distance from river mouth, km (mouth = 0) -25 -20 -15 -10 -5 0 5 TIN : SRP (molar ratio) 1 10 100 1000 10000
11. Chequamegon Bay, 2008 Distance from river mouth, km (river mouth = 0) -25 -20 -15 -10 -5 0 5 Mg (mg L-1) 2 3 4 5 6 7 8 9 10 Distance from river mouth, km (mouth = 0) -25 -20 -15 -10 -5 0 5 13C POC ,‰ -27.4 -27.2 -27.0 -26.8 -26.6 -26.4 -26.2 -26.0 -25.8 -25.6 11 June 2008 POC, mg L-1 0.4 0.6 0.8 1.0 1.2 1.4 1.6
12. 11 Distance from river mouth, km (mouth = 0) -25 -20 -15 -10 -5 0 5 13C POC - 13C POC-CON -2 0 2 4 Chequamegon Bay, 2008 POC - POC CON -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Net POC increase enriched depleted
13. 12 Chequamegon Bay, 2008 13CPOC -32 -31 -30 -29 -28 -27 -26 -25 -24 POM C:N (molar) 0 5 10 15 20 25 river phytoplankton lake phytoplankton vascular plants terrigenous soils
14. 13 Energy (OM) and nutrient processes • POC sink (Fish Creek wetland) and source (mouth) • Fish Creek POM with broad mix of materials, higher C:N • Chl a peak at mouth
15. Fish larvae results Hoffman et al. In Press.
16. 15 Hoffman et al. 2012. 15N results
17. 16 • Fish larvae 15N highly correlated to ln(NH4), ln(TN), and the fraction river water (p<0.0001, n=18) Hoffman et al. 2012.
18. Hoffman et al. 2012. 17
19. 18 Modified from Hoffman et al. In Press. Food web results
20. Food web: Mixing Model Output Hoffman et al. In Press. 19
21. Species Patterns • Larvae that utilized the most OM sources were demersal, associated with periphyton • Larvae that utilized the fewest OM sources were pelagic, associated with phytoplankton • Positive association with terrestrial OM and OM sources is unexpected Hoffman et al. In Press. 20
22. 21 WQ Conclusions •Where waters of varying geochemical composition mix, need a conservative tracer: •Identify nutrient sources and sinks •Characterize POM sources •Explain underlying stable isotope gradients •Evidence for energy and nutrient exchange between lake and river in mixing zones •Lake-river mixing may establish productive region of N-P co-limitation
23. 22 Food Web Conclusions •Fish larvae generally obtained nutritional support from multiple OM sources; most obtained some energy originating outside the region where they were collected •PCA revealed that the number of OM sources was positively correlated with terrestrial OM contribution and greatest in demersal fish larvae captured in the mixed region •These coastal habitats function as a “mosaic”, wherein both adjacent and distant habitats and ecosystems contribute to fish growth during a critical life stage