Lakes: Primary Production, Budgets and Cycling

Biogeochemical Systems -- OCN 401 27 September 2012

Reading: Schlesinger Chapter 7

Outline

1. Temperature / density relationship • Seasonal cycle of lake stratification

2. Carbon and nutrient cycling • Dissolved inorganic carbon speciation • Methods of measuring primary production • Nutrients and productivity / nutrient cycling • Carbon budgets • Nutrient budgets • Nitrogen fixation

3. Lake classification

4. Alkalinity and acid rain effects

1 Temperature / Density Relationship

• Water is densest at 4°C • Both ice and warmer water are less dense • Important implication: ice floats!

• If less-dense water is overlain by denser water, lake overturn occurs

• If denser water is overlain

by less-dense water, Heavier stable stratification occurs

Berner & Berner (1996)

Temperature Structure of a Typical Temperate Lake in Summer

• Epilimnion: warm surface waters • Metalimnion: zone of rapid temperature change (thermocline) • Hypolimnion: cooler, deep waters • Many tropical lakes are permanently stratified • Temperate lakes show seasonal break-down of temperature stratification

2 Seasonal Cycle of Lake Stratification

Early spring (after ice melt):

www.gvsu.edu

Late spring:

3 Summer:

• Dead organic matter sinks from the epilimnion to the hypolimnion

• The decay of this organic matter leads to O2 depletion of the hypolimnion

Early autumn:

4 Autumn:

Winter:

5 Berner & Berner (1996)

Temperature profiles over the year for a typical temperate lake Dashed line represents maximum density at 4 C

Dissolved Inorganic Carbon

(DIC, CO2) Speciation

- 2- DIC = CO2 = [CO2 (aq)] + [HCO3 ] + [CO3 ] Bicarbonate Carbonate ion ion

CO2 (g) CO2 (aq)

+ - CO2 (aq) + H2O H + HCO3

- + 2- HCO3 H + CO3

Thus, CO2 speciation is pH dependent:

6 CO2(aq)

Seawater values shown --- freshwater curves are shifted left ~0.8 pH

aob.oxfordjournals.org

Methods of Measuring Primary Production • Collect water sample • Incubate in clear and dark bottles • Measure either:

– Change in dissolved oxygen (O2) 14 14 - – Production of C-labeled POC (from C-HCO3 additions)

• Calculate results:

– Dark bottle: Respiration (for O2 only) – Clear bottle: Net primary production (P-R)

– Clear bottle - dark bottle: Gross primary production (for O2 only)

7 Lake Productivity is Linked to Nutrient Concentration

Comparison of lakes of the world:

(NPP) • Most lakes appear to be P-limited

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

Nutrient Cycling in Lakes

• Large proportion of P is in plankton biomass; small proportion is “available” (dissolved in lake water)

• Turnover of P in the epilimnion is dominated by bacterial decomposition of organic matter

• When planktonic communities are N-limited, there is a shift from green algae to “blue-green algae”

(cyanobacteria) that fix N2

8 Export Ratio = percentage of PP that sinks to the hypolimnion:

• Export ratio is 10- 50% of NPP

• High hypolimnion concentrations of P are returned to the surface during seasonal mixing

• Turnover of P is incomplete, so some P is lost to sediments

Lake Carbon Budgets

• NPP within the lake is “autochthonous” production

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

• Organic carbon from outside the lake is “allochthonous” production (12% of total carbon inputs)

9 In lakes with lower chlorophyll concentrations (i.e., lower productivity), respiration exceeds production:

• This reflects the increased importance of allochthonous carbon inputs

• 74% of organic inputs are respired in the lake

• 74% of that respiration occurs in the sediment (Many other lakes have more respiration in the water column)

• 7.8% of OC is stored in sediments, similar to rate of accumulation of soil organic matter -- but from a much lower NPP than is typical on lan, due to less efficient anaerobic respiration

10 Lake Nutrient Budgets

• Inputs: precipitation, runoff, N-fixation, groundwater • Loses: sedimentation, outflow, release of gases, groundwater

• Thus, nutrient budgets require an accurate water budget for the system

• Comparison of nutrient residence time with the water residence time gives an indication of the relative importance of internal biotic cycling and physical processes

• Most lakes show a substantial net retention of N and P

• Lakes with high water turnover, however, may show relatively low levels of N and P storage

(Experimental Lakes Area)

Most lakes show a net retention of N and P

11 Lake Nutrient Budgets Nitrogen Fixation

• Lakes with high rates of N fixation show large apparent accumulations of N

• However, the loss of N by denitrification typically far exceeds the input of N by fixation

– Denitrification removes fixed N as N2O and (especially) N2

Lake Classification

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

• Eutrophic lakes are: • high productivity systems • nutrient rich • dominated by nutrients inputs from the surrounding watershed • often shallow and warm

12 Nutrient inputs to oligotrophic lakes are generally dominated by precipitation:

Sedimentation tends to convert oligotrophic lakes into eutrophic ones over time….

• Eutrophication: a natural process by which accumulation of sediment causes 1) lake shallowing and 2) a decrease in the volume of the hypolimnion, increasing O2 drawdown:

Berner & Berner (1996)

• Positive feedback: lower O2, less nutrient retention in sediments, higher productivity, higher rain of organic matter to hypolimnion • Cultural eutrophication: input of nutrients due to human activity accelerates the natural process of eutrophication

13 Alkalinity and Acid Rain Effects

• Alkalinity is roughly equivalent to the imbalance in charge from cations and anions: Alkalinity = cations – anions • The charge imbalance is “corrected” by changes in equilibrium in the DIC system

- 2- - + • Thus: Alkalinity = [HCO3 ] + 2[CO3 ] + [OH ] - [H ]

2- - • Alkalinity increases by processes that consume SO4 , NO3 or other anions, or that release DIC • Alkalinity decreases by processes that consume cations or DIC

+ 2- • Acid rain decreases alkalinity due to additions of H and SO4

14 Sensitivity of Lakes to Acid Rain Effects

Berner & Berner (1996)

Lecture Summary

• Physical properties of water, seasonal climate changes, and the surrounding landscape and geology are major controls on nutrient cycling and NPP in lakes

• Primary production is closely linked to nutrient supply

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

• Eutrophication is a natural process, which can be accelerated by anthropogenic activity

• Acid rain has had profound impacts on some lakes; underlying geology is major factor on lake sensitivity

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