Lakes: Primary Produc1on, Budgets and Nutrient Cycling

Transcription

1 OCN 401-Biogeochemical Systems ( ) Lakes: Primary Produc1on, Budgets and Nutrient Cycling Reading: Schlesinger, Chapter 8 Lakes: Introductory facts and trivia DefiniGon: Lakes are permanent bodies of water surrounded by land! Internal biogeochemical cycling! ConnecGon to the landscape Majority of lakes are in northern temperate zones in formerly glaciated landscapes! PopulaGon centers! Anthropogenic impacts Global lake area is increasing due to dams, creagon of reservoirs 1

2 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 km 3 ) Lecture Outline 1. Seasonal cycle of lake stragficagon 2. Primary ProducGon and Nutrient Cycling in Lakes 3. Lake Budgets (C, N, P) 4. Lake classificagon according to trophic state 2

3 Physical Proper1es of Water: The Temperature-Density Rela1onship Water maximum density at 4 C Ice and warmer water are less dense Important implicagon: 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 stra.fica.on occurs Berner & Berner Fig. 6.1 Lake chemistry and biology are affected by these physical processes. Temperature structure for a typical temperate freshwater lake in summer Epilimnion: warm surface waters; light energy rapidly agenuates with depth Metalimnion: zone of rapid temperature change, or thermocline Hypolimnion: cooler, deep waters Many tropical lakes are permanently stragfied Temperate lakes show seasonal break-down of temperature stragficagon & can mix from top to bogom. 3

4 The Seasonal Cycle of Lake Stra1fica1on Berner & Berner Fig Annual temperature profiles for a typical temperate lake Dashed line represents maximum density at 4 C Freshwater Lake Classifica1on Type Mixing PaKern Example 1. HolomicGc Hypolimnion & Epilimnion mix 1a) DimicGc Mixes 2x/yr Temperate 1b) MonomicGc Mixes 1x/yr - Warm monomicgc Mediterranean - Cold monomicgc Alpine 1c) OligomicGc Mixes irregularly Tropical 1d) Shallow lakes ConGnuous mixing 1e) Very deep lakes 2. MeromicGc Mixes only upper porgon of hypolimnion No mixing between hypolimnion and epilimnion Baikal (depth = 1623 m) Saline lakes Berner & Berner Table 6.2 During stragficagon, deep waters are isolated from the atmosphere and geochemistry can evolve to be quite disgnct from surface waters. 4

5 Phytoplankton: Base of the Aqua1c Food Web DIC P and N can be limigng nutrients in lakes, similar to terrestrial systems. Unlike terrestrial systems, C can also be limigng: - it must equilibrate over a finite surface area, according to CO 2 system reacgons: CO 2 + H 2 O H 2 CO 3 H + + HCO 3-2H + + CO 3 2- Primary Produc1on and Nutrient Cycling Phytoplankton (free-floagng algae) - contribute most of the net primary producgon - are confined to surface waters due to light limitagon NPP depends on external nutrient inputs to epilimnion and regeneragon/recycling Epilimnion is oxic, so organic mager decomposes rapidly by aerobic respiragon Low levels of nutrients are found in surface waters due to efficient phytoplankton uptake Summer Stra1fica1on 5

6 Seasonal Evolu1on of Nutrients in the Epilimnion DIN (µm) DIN (um) DIP (µm) DIP (um) Depth (m) Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct Depth (m) Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct Oct Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct 29-Apr DIN = NO NO NH 4 + DIP = PO 4 3- Ashumet Pond, Cape Cod, MA. Rugenberg and Haupert (unpubl.) Primary Produc1on (PP): Conversion of DIC to Organic-C via Photosynthesis " Collect water samples How do we measure PP? " Incubate water samples in bogles " Two methods for quangfying PP: Oxygen evolu1on: Measure change in dissolved oxygen (O2) Carbon-uptake: Add radiolabeled DIC (e.g., 14 C-HCO 3 ), measure producgon of 14 C-labeled POC 6

7 PP and : The Reac1ons " Water incubated in clear bogles: Oxygen evolugon (photosynthegc O2 producgon) CO 2 + H 2 O O 2 + CH 2 O 14 C-labeled POC (C-uptake during photosynthesis) 14 CO 2 + H 2 O O CH 2 O (eqn. 5.2) " O 2 - evolugon method is complicated by concurrent respiragon: Clear bogle: Net Primary ProducGon (NPP): increase in O 2 is a measure of NPP, photosynthesis in excess of respiragon: NPP = GPP-R Dark bogle: RespiraGon (R): drop in O 2 is a measure of net respiragon: O 2 + CH 2 O CO 2 + H 2 O GPP = NPP + R (e.g., the sum of changes in light & dark bogles) Fate of Primary Produc1on (photosynthe1c POM) Dead pargulate organic mager (POM) sinks into the hypolimnion where it is decomposed by microbial respiragon. Decay within the hypolimnion consumes O 2 - low redox potengal develops - isolated from the atmosphere - no re-oxygenagon As a result of hypolimnion isolagon, and of congnual supply and respiragon of POM: - O 2 is consumed - nutrients build up Water Depth Temperature Photosynthesis: CO 2 + H 2 O O 2 + CH 2 O Oxic respira0on O 2 + CH 2 O CO 2 + H 2 O Sinking POM 7

8 Seasonal Evolu1on of DIN and DIP in the Hypolimnion of a temperate MassachuseKs lake DIN (µm) DIN (um) DIP (µm) DIP (um) Depth (m) Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct Depth (m) Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct Oct Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct 29-Apr Rugenberg and Haupert (unpubl.) Export of POM from the Epilimnion Export rago = percentage of PP that sinks to the hypolimnion. Export rago is 10-50% of NPP in 12 US lakes. Greater fracgonal export in lower producgvity lakes. Impact of POM export on nutrients: o o o Nutrient build-up in hypolimnion Nutrient return to the surface during seasonal mixing Nutrient turnover is incomplete, some C, N & P is lost to sediments Fig % of phytoplankton PP that sinks to hypolimnion (export rago) as a funcgon of lake net primary producgvity; aqer Baines and Pace (1994). 8

9 Consequence of POM Export: Nutrient build-up and O 2 draw-down in Hypolimnion DIP (um) Depth (m) DIP (µm) 29-Apr 8-Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct Jun 7-Jul 9-Aug 14-Sep 12-Oct 26-Oct 29-Apr Lake Produc1vity is Linked to Nutrient Concentra1on 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-limitagon Fig RelaGonship between NPP and phosphate concentragon of lakes of the world (Shindler 1978). 9

10 Nutrient Cycling in Lakes: P vs N limita1on? Natural P input to lakes is small. RetenGon in terrestrial watersheds: vegetagon and soil P associated with soil minerals not bioavailable Nutrient Cycling in Lakes: P vs N limita1on? Natural P input to lakes is small. RetenGon in terrestrial watersheds: vegetagon and soil P associated with soil minerals not bioavailable Large proporgon of P is in plankton biomass; small proporgon is available (dissolved in lake water). 10

11 Nutrient Cycling in Lakes: P vs N limita1on? Natural P input to lakes is small. RetenGon in terrestrial watersheds: vegetagon and soil P associated with soil minerals not bioavailable Large proporgon of P is in plankton biomass; small proporgon is available (dissolved in lake water). P-recycling in the epilimnion is dominated by bacterial decomposigon of organic mager (internal recycling) ProducGon of POM and DOM (DOC, DON, DOP) Nutrient Cycling in Lakes: P vs N limita1on? Natural P input to lakes is small. RetenGon in terrestrial watersheds: vegetagon and soil P associated with soil minerals not bioavailable Large proporgon of P is in plankton biomass; small proporgon is available (dissolved in lake water). P-recycling in the epilimnion is dominated by bacterial decomposigon of organic mager (internal recycling) ProducGon of POM and DOM (DOC, DON, DOP) Phytoplankton and bacteria excrete enzymes to convert DOP to bioavailable PO

12 Nutrient Cycling in Lakes: P vs N limita1on? Natural P input to lakes is small. RetenGon in terrestrial watersheds: vegetagon and soil P associated with soil minerals not bioavailable Large proporgon of P is in plankton biomass; small proporgon is available (dissolved in lake water). P-recycling in the epilimnion is dominated by bacterial decomposigon of organic mager (internal recycling) ProducGon of POM and DOM (DOC, DON, DOP) Phytoplankton and bacteria excrete enzymes to convert DOP to bioavailable PO When N is limigng, algal community shiqs to N 2 -fixing algae, e.g., from green to blue-green algae (cyanobacteria) Role of N-Fixa1on in Epilimnion Biomass (biovolume µm ) Apr-29 Green Algae Blue-green Algae May-13 May-27 Jun-10 Rengefors et al Diatoms Cyanophyceae Dino Cyanophyceae Jun-24 Jul-8 Jul-22 Aug-5 Aug-19 Sep-2 Sep-16 Cyanophyceae Diatoms Chrysophyceae Dinophyceae Chlorophyceae Cryptophyceae Sep-30 Oct-14 Both P & N are depleted in epilimnion. Encourages N-fixing blue-green algal growth Pollutant P encourages bluegreen algal growth, can account for >80% of N-input to phytoplankton. N-input via N- fixagon maintains P-limitaGon 12

13 Role of Sediments in P-Cycling Oxic BoKom Water Anoxic BoKom Water PO 4 Fe(OH) 3 ~PO 4 Fe 2+ + PO 4 anoxic anoxic Role of Sediments in P-Cycling Oxic hypolimnion Oxic surface sediments Oxic BW Anoxic BW PO 4 Fe(OH) 3 ~PO 4 Fe 2+ + PO 4 anoxic anoxic 13

14 Role of Sediments in P-Cycling Oxic hypolimnion Oxic surface sediments Anoxic hypolimnion Reducing sediments Oxic BW Anoxic BW PO 4 Fe(OH) 3 ~PO 4 Fe 2+ + PO 4 anoxic anoxic Lake Carbon Budgets: How and Why? 8.3 IdenGfy and quangfy carbon sources / inputs IdenGfy and quangfy carbon sinks / exports C-sources Evaluate whether system is in balance: steady state IdenGfy pargcular characterisgcs of system C-sinks 14

15 Lake Carbon Budgets: How and Why? 8.3 IdenGfy and quangfy carbon sources / inputs IdenGfy and quangfy carbon sinks / exports C-sources Evaluate whether system is in balance: steady state IdenGfy pargcular characterisgcs of system C-sinks Par1cular Characteris1cs: Autochthonus vs. Allocthonous Carbon NPP within the lake is autochthonous producgon. 8.3 Note importance of macrophytes (i.e., rooted plants), which reflects the extent of shallow water. Organic carbon from outside the lake is allochthonous producgon. 15

16 The Balance between Photosynthesis & Respira1on In lakes with lower chlorophyll (i.e., lower producgvity), respiragon exceeds producgon. This reflects the increased importance of allochthonous carbon inputs. Lake waters are oqen supersaturated with respect to CO 2 relagve to the atmosphere. o Net heterotrophic Fig Mean summergme plankton respiragon (R) and photosynthesis (P) in surface waters of lakes as a funcgon of chlorophyll concentragon, an index of overall lake producgvity (del Georgio & Peters, 1994); from Schlesinger, 2 nd ed. (1997). Par1cular Characteris1cs: Locus & Extent of Respira1on 74.2% of organic inputs are respired in the lake 73.6% of that respiragon occurs in the sediment. (In deeper lakes more respiragon occurs in the water column.) 8% of OC is stored in sediments 8.3 o o similar to terrestrial systems, but from a much lower NPP that is typical on land reflects inefficiency of respira.on, as compared to non-saturated soils. 16

17 Lake Nutrient Budgets Inputs: precipitagon, runoff, N-fixaGon, groundwater Losses: sedimentagon, outlow, 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 biogc cycling - Most lakes show a substangal net retengon of N and P. - Lakes with high water turnover, however, may show relagvely low levels of N and P storage. Many lakes show near-balanced budgets for Mg, Na, Cl - highly soluble elements - non-limigng to phytoplankton One- and Two-box Models for Lakes (a\er Berner and Berner, Figs. 6.5 and 6.6) Input C i F i R P (precipitagon + sedimentagon) R D (dissolugon, regeneragon) DC/Dt = C i F i - C e F o + R D - R P where R S = R P - R D T r = [C] moles C i F i moles/yr F U = rate of water transfer from hypolimnion to epilimnion F D = rate of water transfer from epilimnion to hypolimnion 17

18 Lake Reten1on of P and N Most lakes show net reten1on of N and P Schlesinger, 2 nd ed. (1997) Biogeochemical control on the geochemistry of the system Lake Nutrient Budgets: N fixa1on Lakes with high rates of N fixagon show large apparent accumulagons of N. 80% of N input to Amazon River is via N-fixaGon However, loss of N by denitrificagon >> input of N by N-fixaGon DenitrificaGon removes fixed N as N 2 O and (especially) N C 6 H 12 O NO 3 = 12 N HCO 3 + 2CO H 2 O Anammox is another pathway for removal of fixed N: + - NH 4 + NO 2 = N 2 + 2H 2 O 18

19 Lake Classifica1on by Trophic State Oligotrophic lakes: low producgvity systems nutrient depleted frequently geologically young typically deep, with a cold hypolimnion Eutrophic lakes: high producgvity systems nutrient rich dominated by nutrient inputs from the surrounding watershed oqen shallow and warm may be subject to cultural eutrophicagon Oligotrophic vs. Eutrophic Lakes Nutrient input to oligotrophic lakes is typically dominated by precipitagon. Eutrophic lakes derive nutrients mainly from the surrounding watershed. Schlesinger, 2 nd ed. (1997) Nutrient status is the most useful criterion for disgnguishing oligotrophic vs. eutrophic lakes. SedimentaGon will convert oligotrophic to eutrophic lakes via eutrophicagon - an aging sequence of a lake. 19

20 Eutrophica0on is a natural process by which accumulagon of sediment causes: - lake shallowing - decreased hypolimnion volume - increased O 2 drawdown PosiGve feedback - lower O 2 - less nutrient retengon in sediments - higher producgvity - higher rain of organic mager to hypolimnion Berner & Berner, Fig Cultural eutrophica0on: human acgvity accelerates eutrophicagon. Cultural Eutrophica1on i Berner & Berner (2013) Fig

21 Trophic State of Lakes World-Wide Schlesinger, 2 nd ed. (1997). Lecture Summary / Main Points Physical properges of water and seasonal temperature changes exert profound control on lake! water column structure! nutrient cycling and NPP Primary producgon is closely linked to nutrient supply LimiGng nutrient can evolve over the growing season Nutrient and carbon budgets provide a key means of assessing lake biogeochemical cycling EutrophicaGon is a natural process, which can be accelerated by anthropogenic acgviges 21