Lakes: Primary Produc�On, Budgets and Nutrient Cycling

OCN 401-Biogeochemical Systems (9.27.18)

Lakes: Primary Producon, Budgets and Nutrient Cycling

Reading: Schlesinger, Chapter 8

Lakes: Introductory facts and trivia

• Deﬁnion: Lakes are permanent bodies of water surrounded by land ! Internal biogeochemical cycling ! Connecon to the landscape

• Majority of lakes are in northern temperate zones in formerly glaciated landscapes ! Populaon centers ! Anthropogenic impacts

• Global lake area is increasing due to dams, creaon of reservoirs

1 Lake Baikal, South-Central Russia

• Deepest lake on Earth (1700 m) o Next deepest (1470 m), Tangankika)

• Oldest lake on Earth (25 Ma)

• Holds 20% of total global surface freshwater (23,000 km3)

Lecture Outline

1. Seasonal cycle of lake straﬁcaon

2. Primary Producon and Nutrient Cycling in Lakes

3. Lake Budgets (C, N, P)

4. Lake classiﬁcaon according to trophic state

2 Physical Properes of Water: The Temperature-Density Relaonship

• Water maximum density at 4 °C

• Ice and warmer water are less dense

• Important implicaon: ice ﬂoats!

• If less dense water is overlain by more dense water lake overturn occurs

• If denser water is overlain by less dense water, stable straﬁcaon occurs

• Lake chemistry and biology are aﬀected by these physical processes.

Berner & Berner Fig. 6.1

Temperature structure for a typical temperate freshwater lake in summer

• Epilimnion: warm surface waters; light energy rapidly aenuates with depth • Metalimnion: zone of rapid temperature change, or thermocline • Hypolimnion: cooler, deep waters • Many tropical lakes are permanently straﬁed • Temperate lakes show seasonal break-down of temperature straﬁcaon & can mix from top to boom.

3 The Seasonal Cycle of Lake Straﬁcaon

Berner & Berner Fig. 6.3.

• Annual temperature proﬁles for a typical temperate lake • Dashed line represents maximum density at 4°C

Freshwater Lake Classiﬁcaon

Type Mixing Paern Example

1. Holomicc Hypolimnion & Epilimnion mix

1a) Dimicc Mixes 2x/yr Temperate 1b) Monomicc Mixes 1x/yr - Warm monomicc Mediterranean - Cold monomicc Alpine 1c) Oligomicc Mixes irregularly Tropical 1d) Shallow lakes Connuous mixing Mixes only upper poron of Baikal (depth = 1e) Very deep lakes hypolimnion 1623 m) No mixing between 2. Meromicc Saline lakes hypolimnion and epilimnion Berner & Berner Table 6.2

During straﬁcaon, deep waters are isolated from the atmosphere and geochemistry can evolve to be quite disnct from surface waters.

4 Phytoplankton: Base of the Aquac Food Web

DIC

• P and N can be liming nutrients in lakes, similar to terrestrial systems. • Unlike terrestrial systems, C can also be liming: - it must equilibrate over a ﬁnite surface area, according to CO system reacons: 2 + - + 2- CO2 + H2O H2CO3 H + HCO3 2H + CO3

Primary Producon and Nutrient Cycling

• Phytoplankton (free-floang algae) Summer Straficaon - contribute most of the net primary producon - are confined to surface waters due to light limitaon • NPP depends on external nutrient inputs to epilimnion and regeneraon/recycling • Epilimnion is oxic, so organic maer decomposes rapidly by aerobic respiraon • Low levels of nutrients are found in surface waters due to efficient phytoplankton uptake

5 Seasonal Evoluon of Nutrients in the Epilimnion

DIN (µM) DIP (µM) DIN (uM) DIP (uM) 0 20 40 60 80 100 120 140 0 5 10 15 20 25 30 35 40 0 0

2 29-Apr 2 29-Apr 8-Jun 8-Jun 4 7-Jul 4 7-Jul 9-Aug 9-Aug 6 14-Sep 6 14-Sep 12-Oct 8 12-Oct 26-Oct 8 26-Oct 10 10 Depth (m) Depth 12 Depth (m) Depth 12 14 14 16 16 18 26-Oct 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 18 20 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 29-Apr 20

26-Oct 29-Apr - - + 3- DIN = NO3 + NO2 + NH4 DIP = PO4

Ashumet Pond, Cape Cod, MA. Ruenberg and Haupert (unpubl.)

Primary Producon (PP): Conversion of DIC to Organic-C via Photosynthesis

How do we measure PP?

" Collect water samples

" Incubate water samples in boles

" Two methods for quanfying PP:

– Oxygen evoluon: Measure change in dissolved oxygen (O2)

14 – Carbon-uptake: Add radiolabeled DIC (e.g., C-HCO3), measure producon of 14C-labeled POC

6 PP and : The Reacons " Water incubated in clear boles:

– Oxygen evoluon (photosynthec O2 producon) (eqn. 5.2) CO2 + H2O O2 + CH2O – 14C-labeled POC (C-uptake during photosynthesis)

14 14 CO2 + H2O O2 + CH2O

" O2- evoluon method is complicated by concurrent respiraon: – Clear bole: Net Primary Producon (NPP):

• increase in O2 is a measure of NPP, photosynthesis in excess of respiraon: NPP = GPP-R

– Dark bole: Respiraon (R): drop in O2 is a measure of net respiraon:

O + CH O CO + H O 2 2 2 2

– GPP = NPP + R (e.g., the sum of changes in light & dark boles)

Fate of Primary Producon (photosynthec POM)

Temperature • Dead parulate organic maer (POM) sinks into the Photosynthesis: hypolimnion where it is CO + H O O + CH O decomposed by microbial 2 2 2 2 respiraon. Oxic respiraon • Decay within the hypolimnion O2 + CH2O CO2 + H2O consumes O2 - low redox potenal develops - isolated from the atmosphere

- no re-oxygenaon Water Depth

• As a result of hypolimnion Sinking POM isolaon, and of connual supply and respiraon of POM:

- O2 is consumed - nutrients build up

7 Seasonal Evoluon of DIN and DIP in the Hypolimnion of a temperate Massachuses lake

DIN (µM) DIP (µM) DIN (uM) DIP (uM) 0 20 40 60 80 100 120 140 0 5 10 15 20 25 30 35 40 0 0

26-Oct 29-Apr

Ruenberg and Haupert (unpubl.)

Export of POM from the Epilimnion

• Export rao = percentage of PP that sinks to the hypolimnion.

• Export rao is 10-50% of NPP in 12 US lakes.

• Greater fraconal export in lower producvity lakes.

• Impact of POM export on nutrients:

o Nutrient build-up in hypolimnion o Nutrient return to the surface during seasonal mixing Fig. 8.17. % of phytoplankton PP that sinks to 8.17 o Nutrient turnover is incomplete, some C, N & P is hypolimnion (export rao) as a funcon of lake net lost to sediments primary producvity; aer Baines and Pace (1994).

8 Consequence of POM Export: Nutrient build-up and O2 draw-down in Hypolimnion

DIP (uM) 0 5 10 15 20 25 30 35 40 0

2 DIP (µM) 29-Apr 8-Jun 4 7-Jul 9-Aug 6 14-Sep 12-Oct 8 26-Oct 10

Depth (m) Depth 12

18 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 20

26-Oct 29-Apr

Lake Producvity is Linked to Nutrient Concentraon

Comparison of world lakes:

• Most lakes appear to be P-limited.

• Other factors can be important (e.g., other nutrients, sunlight).

• More recent studies suggest co-limitaon

Fig. 7.10. Relaonship between NPP and phosphate 8.17 8.13 concentraon of lakes of the world (Shindler 1978).

9 Nutrient Cycling in Lakes: P vs N limitaon?

• Natural P input to lakes is small. – Retenon in terrestrial watersheds: vegetaon and soil – P associated with soil minerals not bioavailable

Nutrient Cycling in Lakes: P vs N limitaon?

10 Nutrient Cycling in Lakes: P vs N limitaon?

• Natural P input to lakes is small. – Retenon in terrestrial watersheds: vegetaon and soil – P associated with soil minerals not bioavailable • Large proporon of P is in plankton biomass; small proporon is “available” (dissolved in lake water). • P-recycling in the epilimnion is dominated by bacterial decomposion of organic maer (internal recycling) – Producon of POM and DOM (DOC, DON, DOP)

Nutrient Cycling in Lakes: P vs N limitaon?

11 Nutrient Cycling in Lakes: P vs N limitaon?

• When N is liming, algal community shis to N2-ﬁxing algae, e.g., from green to blue-green algae (cyanobacteria)

Role of N-Fixaon in Epilimnion

• Both P & N are Green Algae Blue-green Algae depleted in epilimnion. 3500 Diatoms Cyanophyceae Dino Cyanophyceae • Encourages N-ﬁxing Cyanophyceae 3000

) Diatoms blue-green algal -6 Chrysophyceae 10

3 growth 2500 Dinophyceae m

µ Chlorophyceae Cryptophyceae • Pollutant P 2000 encourages blue- 1500 green algal growth, can account for 1000 >80% of N-input to Biomass (biovolume Biomass 500 phytoplankton.

0 • N-input via N- ﬁxaon maintains Jul-8 Sep-2 Aug-5 Jul-22 Sep-30 Sep-16 Oct-14 Jun-10 Jun-24 Apr-29 Aug-19 May-13 May-27 P-limitaon Rengefors et al. 2003

12 Role of Sediments in P-Cycling

Oxic Anoxic Boom Boom Water Water

PO4

2+ Fe(OH)3~PO4 Fe + PO4

anoxic anoxic

Role of Sediments in P-Cycling

Oxic hypolimnion Oxic surface sediments

Oxic BW Anoxic BW

PO4

2+ Fe(OH)3~PO4 Fe + PO4

anoxic anoxic

13 Role of Sediments in P-Cycling

Oxic hypolimnion Anoxic hypolimnion Oxic surface sediments Reducing sediments

Oxic BW Anoxic BW

PO4

2+ Fe(OH)3~PO4 Fe + PO4

anoxic anoxic

Lake Carbon Budgets: How and Why?

8.3

• Idenfy and quanfy carbon sources / inputs

• Idenfy and quanfy C-sources carbon sinks / exports

• Evaluate whether system is in balance: steady state

• Idenfy parcular

characteriscs of system C-sinks

14 Lake Carbon Budgets: How and Why?

8.3

• Idenfy and quanfy carbon sources / inputs

• Idenfy and quanfy C-sources carbon sinks / exports

• Evaluate whether system is in balance: steady state

• Idenfy parcular

characteriscs of system C-sinks

Parcular Characteriscs: Autochthonus vs. Allocthonous Carbon 8.3 • NPP within the lake is “autochthonous” producon.

• Note importance of macrophytes (i.e., rooted plants), which reﬂects the extent of shallow water.

• Organic carbon from outside the lake is “allochthonous” producon.

15 The Balance between Photosynthesis & Respiraon

• In lakes with lower chlorophyll (i.e., lower producvity), respiraon exceeds producon.

• This reﬂects the increased importance of allochthonous carbon inputs.

• Lake waters are oen supersaturated with Fig. 7.12. Mean summerme plankton respiraon (R) respect to CO2 relave to and photosynthesis (P) in surface waters of lakes as a the atmosphere. funcon of chlorophyll concentraon, an index of o Net heterotrophic overall lake producvity (del Georgio & Peters, 1994); from Schlesinger, 2nd ed. (1997).

Parcular Characteriscs: Locus & Extent of Respiraon 8.3

• 74.2% of organic inputs are respired in the lake

• 73.6% of that respiraon occurs in the sediment. (In deeper lakes more respiraon occurs in the water column.)

• 8% of OC is stored in sediments

o similar to terrestrial systems, but from a much lower NPP that is typical on land

o reﬂects ineﬃciency of respiraon, as compared to non-saturated soils.

16 Lake Nutrient Budgets • Inputs: precipitaon, runoﬀ, N-ﬁxaon, groundwater

• Losses: sedimentaon, oulow, release of gases, groundwater

• Nutrient budgets require an accurate water budget for the system

• Comparison of nutrient residence me with water residence me indicates importance of internal bioc cycling - Most lakes show a substanal net retenon of N and P. - Lakes with high water turnover, however, may show relavely low levels of N and P storage.

• Many lakes show near-balanced budgets for Mg, Na, Cl

- highly soluble elements - non-liming to phytoplankton

One- and Two-box Models for Lakes (aer Berner and Berner, Figs. 6.5 and 6.6)

Input

CiFi

RP (precipitaon + sedimentaon)

RD (dissoluon, regeneraon)

DC/Dt = CiFi - CeFo + RD - RP FU = rate of water transfer from hypolimnion to epilimnion where RS = RP - RD FD = rate of water transfer from epilimnion to hypolimnion Tr = [C] moles ÷ CiFi moles/yr

17 Lake Retenon of P and N

Schlesinger, 2nd ed. (1997) • Most lakes show net retenon of N and P

• Biogeochemical control on the geochemistry of the system

Lake Nutrient Budgets: N ﬁxaon

• Lakes with high rates of N ﬁxaon show large apparent accumulaons of N.

– 80% of N input to Amazon River is via N-ﬁxaon

• However, loss of N by denitriﬁcaon >> input of N by N-ﬁxaon

• Denitriﬁcaon removes ﬁxed N as N2O and (especially) N2. - - 5 C6H12O6 + 24 NO3 = 12 N2 + 24 HCO3 + 2CO2 + 18H2O

• Anammox is another pathway for removal of ﬁxed N: + - NH4 + NO2 = N2 + 2H2O

18 Lake Classiﬁcaon by Trophic State

• Oligotrophic lakes: – low producvity systems – nutrient depleted – frequently geologically young – typically deep, with a cold hypolimnion

• Eutrophic lakes: – high producvity systems – nutrient rich – dominated by nutrient inputs from the surrounding watershed – oen shallow and warm – may be subject to “cultural eutrophicaon”

Oligotrophic vs. Eutrophic Lakes

• Nutrient input to oligotrophic lakes is typically dominated by precipitaon. • Eutrophic lakes derive nutrients mainly from the surrounding Schlesinger, 2nd ed. (1997) watershed. • Nutrient status is the most useful criterion for disnguishing oligotrophic vs. eutrophic lakes.

• Sedimentaon will convert oligotrophic to eutrophic lakes via eutrophicaon - an aging sequence of a lake.

19 • Eutrophicaon is a natural process by which accumulaon of sediment causes: - lake shallowing - decreased hypolimnion volume - increased O drawdown 2 • Posive feedback

- lower O 2 - less nutrient retenon in sediments - higher producvity - higher rain of organic maer to hypolimnion

Berner & Berner, Fig. 6.8.

• Cultural eutrophicaon: human acvity accelerates eutrophicaon.

Cultural Eutrophicaon

• Berner & Berner (2013) Fig. 6.10.

20 Trophic State of Lakes World-Wide

Schlesinger, 2nd ed. (1997).

Lecture Summary / Main Points

• Physical properes of water and seasonal temperature changes exert profound control on lake

! water column structure

! nutrient cycling and NPP

• Primary producon is closely linked to nutrient supply

• Liming nutrient can evolve over the growing season

• Nutrient and carbon budgets provide a key means of assessing lake biogeochemical cycling

• Eutrophicaon is a natural process, which can be accelerated by anthropogenic acvies