A Modeling Analysis of the Clear Water Phase CE Surface Water Quality Modeling Case History

Chlorophyll Secchi Disc A widespread phenomenon, following a pattern … A distinct period of clear water typical of spring algal succession in many temperate mesotrophic and eutrophic lakes. Lampert, W. et al Phytoplankton control by grazing zooplankton: A study on the spring clear water phase. Limnology and Oceanography.

The PEG “Model” “ … it is assumed that events in the plankton are neither random not chaotic but that there is a seasonal development which is a predictable consequence of previous events.” The Plankton Ecology Group, a working body associated with SIL, sought to construct a word model of the seasonal events which occur in the phytoplankton of an idealized ‘standard’ lake. Sommer et al. 1986

… well described by the Phytoplankton Ecology Group (PEG) Model PEG 1. Towards the end of winter, nutrient availability and increased light permit unlimited growth of the phytoplankton. A spring crop of small, fast- growing algae such as Cryptophyceae and small centric diatoms develops. Data for Onondaga Lake Auer et al. 1990

… well described by the Phytoplankton Ecology Group (PEG) Model Data for Onondaga Lake Auer et al Spada et al PEG 4. Herbivore populations increase exponentially up to the point at which their density is high enough to produce a community filtration rate, and thus cropping rate, that exceeds the reproduction rate of the phytoplankton.

… well described by the Phytoplankton Ecology Group (PEG) Model Data for Onondaga Lake Auer et al PEG 5. As a consequence of herbivore grazing, the phytoplankton biomass decreases rapidly to very low levels. There then follows a ‘clear-water’ equilibrium phase which persists until inedible algal species develop in significant numbers.

… well described by the Phytoplankton Ecology Group (PEG) Model Data for Onondaga Lake Spada et al PEG 6. Herbivorous zooplanktonic species become food-limited and both their body weight per unit length and their fecundity declines. This results in a decrease in their population densities and biomasses.

… well described by the Phytoplankton Ecology Group (PEG) Model Data for Onondaga Lake Spada et al PEG 7. Fish predation accelerates the decline of herbivorous planktonic populations to very low levels and this trend is accompanied by a shift towards smaller average body size amongst surviving crustaceans.

… well described by the Phytoplankton Ecology Group (PEG) Model Data for Onondaga Lake Auer et al PEG 8. Under conditions of reduced grazing pressure and sustained non-limiting concentration of nutrients, the phytoplankton summer crops start to build up. The composition of the phytoplankton becomes complex both due to the increase in species richness and to the functional diversification into those species available to filter-feeders and those only consumed by specialist feeders.

… well described by the Phytoplankton Ecology Group (PEG) Model Data for Onondaga Lake Auer et al PEG 10. From this time onward, the algal growth becomes nutrient-limited and this prevents an explosive growth of ‘edible’ algae. Grazing by predator-controlled herbivores balances the nutrient-limited growth rate of edible algal species.

modeling … from words to equations PEG 1…, PEG 4…, PEG 5…, PEG 7…, PEG 8… AQUATOX 2.0 Park, R.A., Clough, J.S. and M. Coombs Wellman AQUATOX: Modeling Environmental Fate and Ecological Effects in Aquatic Ecosystems. Release 2. U.S. Environmental Protection Agency, Office of Water, Washington, DC.

edible algae: flagellate greens small diatoms cryptomonads inedible algae: large diatoms large greens cyanobacteria dinoflagellates ceaspub.eas.asu.eduserc5.si.eduwww.biology.mcgill.ca biodidac.bio.uottawa.ca modeling … organisms

cladoceran: Daphnia planktivore: gizzard shad pref edible = 1 pref inedible = 0 pref cladoceran = 1

modeling … species composition sm diat crypto dino cyano lg gr edible inedible Data for Onondaga Lake UFI; Cliff Siegrfried

modeling kinetic coefficients edible algae K p = mg/L T opt = 16 °C C 0 = 0.4 mg/L inedible algae: K p = mg/L T opt = 25 °C C 0 = 0.05 mg/L Values for other coefficients used in the calibration process (P max, K resp, K mort, K settle ) were identical for both groups.

modeling … nutrients Set initial conditions and loads to achieve saturation. Nitrogen (mgN∙L -1 ) Phosphorus (mgP∙L -1 )

modeling … temperature Incident Light (ly∙d -1 ) Latitude-specific seasonal pattern. Temperature (°C) Latitude-specific seasonal pattern.

modeling … optics Secci disc transparency is estimated from the model-calculated extinction coefficient, based on a polynomial published by Effler et al. (1996): The extinction coefficient is calculated as the sum of partial extinction coefficients provided by Effler et al. (1996):

modeling … transparency Set detritus to achieve ‘clearwater’ transparency. Secchi Disk (m)

modeling … target data sets Data for Onondaga Lake Spada et al Maximum Secchi Disc (m) return of large-bodied Daphniids relapse

Oh … now we get to see the snake eat the rabbits!

1985 data set... chlorophyll and transparency M J J A S O N

1985 calibration … chlorophyll M J J A S O N dominant

1985 calibration … transparency M J J A S O N

1985 calibration … zooplankton absent in 1985

1999 data set … chlorophyll and transparency A M J J A S O

1999 data set … daphniids A M J J A S O daphniids G max = 1.6 g/g·d T opt = 20 °C C 0 = 0.01 mg/L Other coefficients used in the calibration process included K resp and K mort.

1999 calibration … daphniids A M J J A S O

1999 performance … chlorophyll A M J J A S O

1999 performance … transparency A M J J A S O

1999 performance … species composition model data

2003 simulation …planktivore added cladocerans essentially eliminated much slower attenuation of edible forms dominated by edible algae

2003 simulation …planktivore added absence of clear water phase

What about the rabbits that got away?

… rabbits running, Part 1: time-variable TSS TSS = 3 mg/L TSS = 0.3 mg/L The base case TSS is ramped down from 3 mg/L to 0.3 mg/L during the clearing event, Neither level, held constant permits successful simulation of transparency over the season. This suggest that non-specific grazing (tripton consumption) may play a role in the magnitude of the transparency increase observed in clearing events. time-variable TSS

… rabbits running, Part 2: Aphanizomenon A M J J A S O … the model fails to capture a late June crash in chlorophyll

… rabbits running, Part 2: Aphanizomenon the population crashing was a genus of cyanobacteria, Aphanizomenon, an inedible form. The crash freed resources for two edible groups (small diatoms and cryptomonads) …

… rabbits running, Part 2: Aphanizomenon and a second daphniid peak resulted … A M J J A S O

… rabbits running, Part 2: Aphanizomenon A M J J A S O … bottoming out the chlorophyll

… rabbits running, Part 2: Aphanizomenon A M J J A S O … an yielding a second clearing event

… we have our ways to deal with running rabbits “Pay no attention to the modeler behind the curtain.”

Conclusions The PEG word model and the AQUATOX software package provide a framework for the successful simulation of the clearing event phenomenon and for phytoplankton-transparency relationships in non-clearing event years. Certain features of the simulation, particularly the demise of cyanobacteria populations and the role of non-specific grazing in driving clearing events, are not well understood … reminding us that nature remains the master modeler.