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 15 .