ISEO: Improving the lake Status from Eutrophy towards Oligotrophy
University of Parma
Nutrient loads, factors affecting their availability and response of submerged vegetation Main activities
WP1: to estimate N and P potential loads of the different anthropic activities in the watershed
to quantify nutrients (P, N and Si) loads to lake Iseo, evaluate how their magnitude and bioavailability are affected by hydrological conditions
to check (and improve) the accuracy of P determination by the in situ auto analyzer
WP2: to quantify nutrients (P, N and Si) concentrations in waters discharged by sewer overflows
to evaluate the functioning of the littoral areas as a buffer of the external nutrients loads
to map the extension and composition of submerged macrophytes meadows and their nutrients content and how they change in relation to external pressures Net anthropogenic P and N input to watersheds: a comparison
350 4000
300 3000 250
-1
-1
y
y
-2 -2 200 2000 150
kg P km kg 100 N km kg 1000 50
0 0
f. toce f. toce f. sarca f. mera f. mera f. sarca f. caffaro f. caffaro f. oglio imm f. ticino imm f. adda imm f. oglio imm f. adda immf. ticino imm f. chiese imm f. chiese imm
Net anthropogenic phosphorus Net anthropogenic nitrogen input input = 562 t P y-1, = 6325 t N y-1 (areal load of 314 kg P km-2 y-1) (areal load of 3526 kg N km-2 y-1)
average Po river watershed: 800 kg average Po river watershed: 8000 N km-2 y-1 kg N km-2 y-1 all fluxes are tons/y input = 562 t P y-1
P-fert P-feed P-food P-det
32 328 149 18
3 185 103 237 512 255
126 350 145 Main activities
WP1: to estimate N and P potential loads of the different anthropic activities in the watershed
to quantify nutrients (P, N and Si) loads to lake Iseo, evaluate how their magnitude and bioavailability are affected by hydrological conditions
to check (and improve) the accuracy of P determination by the in situ auto analyzer
WP2: to quantify nutrients (P, N and Si) concentrations in waters discharged by sewer overflows
to evaluate the functioning of the littoral areas as a buffer of the external nutrients loads
to map the extension and composition of submerged macrophytes meadows and their nutrients content and how they change in relation to external pressures Why Silica
• essential element for many aquatic primary producers;
• control on communities composition;
• changes in Si availability in relation to P and N can trigger harmful algal blooms;
• Si transport has been largely considered a geochemical process, mainly regulated by chemical weathering and hydrology;
• lakes are biogeochemical reactors;
• relatively little is known about Si biogeochemistry in lakes;
• little is known about the role of the littoral zone on Si cycling. AIMS
to analyse factors controlling silica transport and retention. More specifically: AIMS
to analyse factors controlling silica transport and retention. More specifically: to quantify the amount and composition of incoming silica loads to the lake and Si lake retention Si Si AIMS
to analyse factors controlling silica transport and retention. More specifically:
to evaluate how the lake to quantify the amount and influence nutrient ratios composition of incoming silica loads (N:P:Si) along the to the lake and Si lake retention hydrographic network Si Si N N P P AIMS
to analyse factors controlling silica transport and retention. More specifically:
to evaluate how the lake to quantify the amount and influence nutrient ratios composition of incoming silica loads (N:P:Si) along the to the lake and Si lake retention hydrographic network Si Si N N P P
to quantify silica accumulation and recycling in the monimolimnion of the lake AIMS
to analyse factors controlling silica transport and retention. More specifically:
to evaluate how the lake to quantify the amount and influence nutrient ratios composition of incoming silica loads (N:P:Si) along the to the lake and Si lake retention hydrographic network Si Si N N P P
to quantify silica accumulation and recycling in the littoral zone in relation to the to quantify silica accumulation activity of primary and recycling in the producers monimolimnion of the lake MATERIAL AND METHODS
estimation of nutrients load:
Water samples were collected in three site: two main inlets and the outlet.
Dissolved parameters: dissolved silica (DSi), soluble reactive phosphorus (SRP) and + dissolved inorganic nitrogen (DIN = NH4 + - NO3 ), dissolved organic phosphorus and nitrogen. Suspended material: amorphous silica (ASi), particulate phosphorus (PP) and particulate nitrogen (PN).
Loads calculated using as the product of the discharge weighted mean concentration by the mean annual discharge I Inlet of the 3 years O Outlet (푄푖∗퐶푖) 퐿 = ∗ 푄 ∗ 푘 푄푖 푡표푡 MATERIAL AND METHODS
Lake Inlet Lake Outlet 160 Oglio river Industrial channel Sarnico 140 120 100
-1
s
3 80 m 60 40 20 0
04/16 08/16 12/16 04/17 08/17 12/17 04/18 08/18 04/16 08/16 12/16 04/17 08/17 12/17 04/18 08/18 04/16 08/16 12/16 04/17 08/17 12/17 04/18 08/18
A total of 30 water samples were collected at each station from April 2016 to Novembre 2018 MATERIAL AND METHODS
Silica sedimentation rates and release from deep sediments:
Quantification of silica sedimentation rates analyzing (ASi) in the particulate material collected with the sedimentation traps (thanks Michael)
Intact sediment cores were collected at the deeper layer (250 m c.a.) and incubated for flux measurement. (many thanks Michael)
Quantification of silica in porewater (DSi), and sediment (ASi). MATERIAL AND METHODS
nutrients accumulation and recycling in the littoral zone:
Three different littoral habitats (depth < 5m, rocky with epilithon, sediments with microalgae and with macrophytes) were sampled in 3 dates from May to September 2017.
Intact sediment cores or rocky substrates were collected and incubated.
Quantification of Si fluxes under light/dark cycles. Silica pools: in water (DSi), primary producers biomass and surficial sediment (BSi). MATERIAL AND METHODS
nutrients accumulation and recycling in the littoral zone:
Three different littoral habitats (depth < 5m, rocky with epilithon, sediments with microalgae and with macrophytes) were sampled in 3 dates from May to September 2017.
Intact sediment cores or rocky substrates were collected and incubated.
Quantification of Si fluxes under light/dark cycles. Silica pools: in water (DSi), primary producers biomass and surficial sediment (BSi). Si external loads
15000 TSi Baseflow Highflow
400
15000 TSi Baseflow Highflow
10000
300
10000
kg d-1 kg
200
mg L-1 mg
5000
kg d-1 kg
100
5000
0
0
1Main River 2 3 Channel 4 1 Baseflow 1 Highflow Baseflow Highflow
0
Main River Channel 1 Baseflow 1 Highflow Baseflow Highflow kg d-1
0 5000 kg10000 d-1 15000
0 5000 10000 15000
1 1 Baseflow mg L-1 Si RESULTS TSi
0 100 200 300 400 external 1 1 Baseflow TSi Main River 1 1 Highflow 1 Main River loads 1 1 Highflow 2 Baseflow Baseflow Channel 3 Channel Highflow Highflow Baseflow Highflow Highflow Baseflow 4
% 100 20 40 60 80 0 Main River Highflow Baseflow
% 100 20 40 60 80 0 Channel Baseflow ASi BSi BSi DSi Highflow Baseflow Highflow Si, N and P loads and retention
Silica Nitrogen Phosphorus 3500 350 Particulate 3000 Dissolved 300
2500 250
-1 2000 200
t year t 1500 150
1000 100
500 50
0 0 Input Output Input Output Input Output
annual retention (%) annual retention (%) annual retention (%) 63% 14% 68%
Data from January 2016 to December 2018 Influence of lake biogeochemical processes on Si, P and N ratios
P:Si = 0.025 25 Inlet Outlet Si is always in limited supply 20 compared to N. Winter Summer During the summer period Si 15 limitation increases in outflowing waters compared to both N and 10 P.
N:Si (mol:mol)
5
N:Si = 0.425 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 P:Si (mol:mol) Influence of lake biogeochemical processes on Si, P and N ratios
P:Si = 0.025 25 Inlet Outlet Si is always in limited supply 20 compared to N. Winter Summer During the summer period Si 15 limitation increases in outflowing waters compared to both N and 10 P.
N:Si (mol:mol)
5
N:Si = 0.425 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 P:Si (mol:mol) where does the retained Si go?
1000
500
0 Fluxes
-1 -500 Net sink Net
ton Si y Si ton -1000
Sedimentation Lake retention Lake -1500
-2000
-2500 Si recycling in the monimolimnion
5000 1000
500 4000 0 Fluxes
-1
3000 -1 -500 Net sink Net
ton Si y Si ton -1000
µg Si L µg Si 2000
Sedimentation Lake retention Lake -1500
1000 -2000 1.5 mg Si m-2 h-1 0 -2500 0 20 40 60 h Si accumulation in the monimolimnion
1000 Si Epilimnion 500 2240 tons of Si 0 - 20 0 Fluxes
Hypolimnion -1 -500
20 - 90 9900 tons of Si Net sink Net
ton Si y Si ton -1000
Depth (m)
Sedimentation Lake retention Lake -1500 90 - 251 -2000 Monimolimnion 12500 tons of Si -2500
0.0 2.0 4.0 6.0 8.0 10.0 mg Si l-1
ARPA: 2009 - 2016 Si fluxes in the littoral zone
400 400 4
200 200 2
1
-
d
2 -
0 0 0 mg mg m
-200 -200 -2
DRSi -400 -400 -4 periphyton macrophytes sediment periphyton macrophytes sediment periphyton macrophytes sediment Si, N and P fluxes in the littoral zone
400 400 4
200 200 2
1
-
d
2 -
0 0 0 mg mg m
-200 -200 -2
DRSi DIN SRP -400 -400 -4 periphyton macrophytes sediment periphyton macrophytes sediment periphyton macrophytes sediment Silica contents in primary producers of littoral zone
Aboveground Roots
5 5 0.5 Si N P 4 4 0.4
3 3 0.3 -2 2.7%
g m g 2 2 0.2 0.6% 1 1 0.1 n.d. 0 0 0.0 A B C A B C A B C
0.05 - 1.0% in Egeria densa 0.05 - 0.9% in Nuphar lutea 2.4 - 4.6% in Cladophora glomerata + diatoms
(Malkin et al., 2009; Shoelynk et al., 2012; Shoenlynk & Struyf., 2016) Nutrients content and stoichiometry in primary producers
Aboveground Roots
5 5 0.5 Si N P 4 4 0.4
3 3 0.3
-2
g m g 2 2 0.2
1 1 0.1 n.d. 0 0 0.0 A B C A B C A B C
Site N:Si P:Si A filamentous algae 0.45 (± 0.11) 0.01 (± 0.01) B aboveground 6.71 (± 2.56) 0.21 (± 0.12) B roots 12.22 (± 5.93) 0.31 (± 0.11) Flux stoichiometry
N vs Si P vs Si 200 4 N:Si = 0.42 P:Si = 0.025
100 2
-1
-1
h
h
-2
-2 0 0
µmol P m µmol µmol N m µmol -2 -100
-4 -200 -100 0 100 200 300 -100 0 100 200 300 µmol Si m-2 h-1 µmol Si m-2 h-1 Flux stoichiometry
N vs Si P vs Si 200 4 N:Si = 0.42 P:Si = 0.025
100 deep sed 2
-1
-1
h
h
-2
-2 0 0
µmol P m µmol µmol N m µmol -2 -100
-4 -200 -100 0 100 200 300 -100 0 100 200 300 µmol Si m-2 h-1 µmol Si m-2 h-1 Concluding remarks
Lake Iseo is a net and efficient sink of Si (2000 tons Si y-1) and phosphorous (86 tons P y-1). Efficient Si retention influences nutrients proportions along the hydrographic network especially during the summer period, resulting in an increase of N:Si and P:Si molar ratio in the Oglio river.
The retained Si is mainly deposited to the lake bottom (1634 tons Si y-1) and the Si released from the same sediments (523 tons Si y-1) is trapped into the monimolimnion.
On average the littoral zone is a net source of Si to the water column during the summer season. This is an important function as Si recycling is a source to phytoplankton.
However littoral primary producers influence differently water-sediment Si fluxes. The littoral zone with sediment is a net Si source to the water column (sites VS = 364±119 ton Si y-1 and BS = 111±45 ton Si y-1 ) while rocky shores are a net Si sink (- 59±15 ton Si y-1 ). 1 first level degree thesis, Natural and environmental Sciences Ferrari Alessandro, Water quality in the Oglio river flowing into the Iseo Lake.
2 second level degree thesis Ceccon Silvia, Origin and transfer of phosphorus and nitrogen loadings in the Lake Iseo watershed.
Cristini Domiziana, Evaluation of nitrogen and phosphorus loads and benthic metabolism in the littoral zone of Lake Iseo.
1 PhD thesis in Ecology Scibona Alessandro Influence of hydrology and primary producers activity on silica biogeochemistry in shallow aquatic environments Congresses
Nizzoli D, Scibona A, Bolpagni R, Longhi D, Cristini D, Viaroli P. Silica dynamics in a subalpine lake: from external loads to in lake processes. XXVIII Convegno Nazionale della Società Italiana di Ecologia Cagliari 12-14 settembre 2018.
Scibona A., Nizzoli D., Cristini D., Longhi D., Bolpagni R., Viaroli P. Silica regulation and nutrients stoichiometry in a subalpine lake: from external loads to in lake processes. XXIV Congresso AIOL. Bologna 5-7 giugno 2019
In preparation
Scibona A., Nizzoli D., Cristini D., Bolpagni R., Viaroli P. Influence of different primary producers on silica tranformations and nutrients stoichiometry in a subalpine lake (lake Iseo). Waters
Scibona A., Nizzoli D., Hupfner M., Pilotti M., Viaroli P. Silica load and nutrient stoichiometry in a meromitic lake (lake Iseo, italy). Biogeochemistry
N and P loads to lake Iseo
DRSi BSi Tsi TP SRP TDP PP TN NH4 NO3 TDN PN
Canal 1399 78 1477 41 11 14 29 885 32 652 825 59
Oglio 1227 340 1568 85 17 22 62 1232 72 859 1050 174
Input 2626 418 3044 126 28 35 91 2117 104 1510 1875 233
Output 980 162 1142 40 8 18 22 1828 94 1095 1674 153
Retention 1646 256 1902 86 20 17 69 289 10 415 201 80
% 63 61 62 68 71 49 76 14 10 27 11 34