Outline

• Loading –concentrations• littoral-open water interactions• cyanobacteria and nutrients , N:P• nitrogen-macrophytes• temperature cyanobacteria• diffuse loading problem

• focus on Lake Taihu…

Plake= Pin/(1+tw), tw = y-1

After 10 years

Chen et al, 2003

P-lake/P-in*100

Hydraulic retention time (years

P-lake=P-in/(1+retention time**0.5)

Lake Taihu retetention estimated:39% - ”measured”35% (Qin, 2007)

Zhu et al, 2006

Bioavailable P

Bioavailable P typically >500-2000 in DK lakes

Lake Arresø

Area 40 km2

% of country area 0.09

Mean depth 3.1 m

Max depth 5.9m

Tw 3.5 y

Lake Taihu

Area 2338 km2

% of country area0.02!!!

Mean depth 1.9 m

Max depth 4.5m

tw 0.8 y

Hydraulic retention time:3.5 years

Simple dilutionThree retention times (10.5 years) reduced by 95%

Jeppesen et al ,2007

Jeppesen et al ,2007

Resuspenion does not prevent improvements-even in relatively large shallow lakes

Fraction of suspended solidsmade up by detritus and inorganic SS

Seasonal phosphorus concentrations (relative to winter values) in 265 shallow Danish lakes with different P levels.

Seasonal changes in P concentration

Seasonal P concentration and retention in 14 shallow Danish lakes with reduced P

Phosphorus release mechanisms/factors

•Physical: temperature• Chemical• Biological

Thickness of oxidised surface layer in four lakes at 7, 14 and 21 oC

Cumulative P release from four lakes at 7, 14 and 21 oC.Based on Jensen et al. (1992)

• P release increases at rising temperatures due to increased mineralization and oxygen/nitrate consumption, thus diminishing the oxidised surface layer.

Temporal variation in P

Model

dPV/dt = (Q/V)*[Pin]-SED+REL waterdPS/dt = SED-REL-IMM sediment

SED = bS*PV/ZREL = bF*(tF^(TEMPV-20))*PSIMM = bI*PS

parameters from the Danish model: bS=0.0695, bF=0.000468, tF=0.0867, bI=0.0000923. - 16 lakes-many years

PV og PS [g/m^2], rates [g/m^2/d].

Time step 1 month

TP (mg P l 1)

0.00.10.20.30.4

Time (days)

0 720 1440 2160 2880

TP (mg P l 1)

0.00.51.01.52.02.5

Time (days)

0 720 1440 2160 2880Borup

Examples of the seasonal P-model

Useful for Lake Taihu?Maybe if including the hydrodynamics

Lake Engelsholm

Before fish removal: 1989-1993

After fish removal : 1994-1999

Søndergaard et al,2003

P dynamics

Simple models useful?

Complex models taking hydrodynamicsinto account better?

Sediment –water interactions – how to do realistic experiments

Spatial and temporal variability

Role of N for P release, role of macrophytes (bays), role of fish

What happens when N is passing through a lake

log (NTOT) =

-0.80±0.08 +0.82±0.03 log (TNOTin) +0.13±0.03 log (Z) -0.20±0.02 log (TW) r2 = 0.75

log (NO) = -2.17±0.08 +0.95±0.03 log (NOin) +0.53±0.04 log (Z) -0.43±0.02 log(TW) r2 = 0.92

lake intercept inlet mean depth retention time

Lake Taihu: 1.9 m mean depth, 5month retention time

prediction 56% loss - ”measured” 49%

N-lakerelated to:-Depth-TW-N-in-Temp

Loading

Retention

Retention (%)

Transformations within the nitrogen cycle in shallow aquatic systems(Slide modified from version of Soonmo An)

PONNO3

-

NO2-

NH4+

DON

N2

N2O

PON

DON

NO3-

NO2-

NH4+

N2

N2ONO3-

Sinking

DENITRIFICATION

Fixation

Assimilation

Excretion

Sediment

Water column

DNRA

N-dynamics (chemical perspective)

Mark in Actionat Lake TaihuChina

McCarthy et al,2007

McCarthy et al,2007

N dynamicsSimple models useful?

Complex models taking hydrodynamicsinto account better?

Sediment –water interactions – how to do realistic experiments

Spatial and temporal variability

Role of macrophytes (bays), role of fish

Lake water changes during 10 days at changing wind speed (from 0-2 to 5-7 m/s to 2-3 m/s) in Western Stadil Fjord, Denmark (450 ha, mean depth: 0.8 m).

•Physical: resuspension by wind/benthivorous fish

•Resuspension increases turbidity and sediment-water interactions, particularly in very shallow lakes.

•Submerged macrophytes and floating leaved plants may put a damper on the impact of resuspension. (Huang et al,2007)

Pelagic- littoral zone interaction , Bay-main lake interactions

Huang et al, 2007 - resuspension 50% of open water in Trapa beds.

Wang ,2007 - Higher concentration of nitrate in the reedzone than 200 m away – indicating that trapped nutrients are released from in here and trabsported to theopen water.

Littoral-open water interactions (trap-release) an important research area . Redsistribution of sediment as well.

Local removal of sediment a relastic symptom treatment?

Diatoms

Silicate

Cyanobacteria

green algae

Lake ArresøJeppesen et al ,2007

Chen et al,2003

Water clarity increases not leastafter zooplanktonbiomass and body size increases

Shallow Danish lakes

N-fixing Cyanothennon-N fixing Cyanothen Green algae

with increasing TP

Jensen et al, 1994

N fixers at intermediate TN:TPnot low TN:TP

Green algae and not N fixingcyanoes at the lowestinorganic N

Jensen et al, 1994

Søndergaard et al, 2005

Deep

Shallow

Deep

High N input-- Enhances or reduces importance of cyanobacteria?

or is P affinity the important factor ?

is N:P at all of importane in shallow lakes?

Should we focus on nutrient release /turnover insteadand on benthic pelagic coupling

Changes in water temperature in Danish survey lakes 1989-2005

simpel lineær regression

Model: log(cyanobacteria biomass) = log(TP), log (mean depth), log(wat. temp.)

Jeppesen et al, in prep.

Model: log(cyanobacteria biomass) = log(TP), log (mean depth), log(wat. temp.)

Jeppesen et al, in prep.

Climate warming will likely enhance the risk of cyanobacteria dominance and the duration of blooming during the year

Gonzales Sagrario et al, FWB , 2005

Biomass submerged plants

last day

one with plants

Perihyton and phytoplankton growth stimulated by N thereby out-shading the submerged macrophytes

N threshold not a constant!!!various with fish density and probably climate

Comparative studies

Uruguay (30°-35°S)

Denmark (55°-57°N)

Meerhoff et al, in press,submittedand on the road

Littoral communities structure

High N may lead to losses of macrophytes in temperate lakesat intermediate high TP

How about Lake Taihu plant communities?

Warmer winters than in DK , much warmer summers

Role of fish different or?.

A challange with diffuse loading!!!

Nett accumulation of phosphorus in Danish agriculture

(Dalgaard & Kyllingsbæk, 2003)

TN and TP in Danish lakes

Søndergaard et al, in press

Søndergaard et al, in press

Shallow

all seasons, but see spring May, June ,autumn

TP TN

Slope for log chla = x log TP + y log TN

month

TP

TN

FLORIDA - DENMARK

month

X Y

Weyhenmeier et al, in press