Freshwater C and Nutrients

carbon hot spots

lakes and rivers on the Earth are carbon hotspots but they don’t cover much of the earth’s surfaces - many of them but most are small - they fundamentally change the way the land interacts with the oceans

what commonly limits production in freshwater

N and P

what mediates N cycling

a more diverse array of microbial processes than is P cycling - aquatic food webs are different than terrestrial food webs because the primary producers are different

freshwaters are insignificant in terms of surface areas

freshwater bodies cover only small fraction of Earth’s surface but play a crucial role in biogeochemical cycles, despite their size

the insignificance of inland waters

compared to oceans, inland waters are minor but disproportionately significant in carbon processing and nutrient cycling

• for total global water, 96.5% oceans, and 2.5% freshwater

  • for that freshwater: 30.1% groundwater, and 68.6% glaciers and ice caps

surface area of the Earth

total earth surface is approximately 510 million km2

• oceans are 71% and land is 29%

how many lakes are there

millions of lakes globally, with an estimated number exceeding 117 million

• many small lakes contribute significantly to biogeochemical processes despite their limited surface area

lakes process C efficiently

lakes are active sites for carbon cycling, processing large amounts of organic carbon through respiration, burial, and outgassing

the pipe model

conceptualizes freshwater systems as “pipes” where carbon and nutrients enter, or processed, and either accumulate or exit - highlights lakes role in retaining and transforming materials before they reach the ocean

the passive pipe

freshwater systems are seen as simply conduits, passively transporting carbon from land to oceans without significant internal processing

the active pipe

inland waters actively retain, and even emit, carbon

small lakes and organic carbon accumulation

small lakes have higher per-area rates of organic carbon accumulation than large lakes - they serve as critical sites for carbon burial, reducing atmospheric CO2 levels

carbon burial in perspetive

freshwater systems bury carbon at rates comparable to or exceeding ocean sediments. despite limited global coverage, lakes and reservoirs contribute significantly to carbon sequestration. Lakes and reservoirs bury carbon at rates up to 50x greater than the open ocean. Inland water store substantial amount of organic carbon long-term

how much carbon do small lakes bury

72 g-2m-1

how much carbon do reservoirs bury each year

400 g-2m-1

how much c does total inland bury

298 Tg/yr

all freshwater as a source of CO2

1.8 Pg/yr another 0.3 from lakes - most freshwater systems are net heterotrophic, meaning they release more co2 than they absorb - high respiration rates contribute to atmospheric CO2 emissions

lakes fundamentally change the way land interacts with the oceans

lakes, rivers, and reservoirs regulate the movement of carbon, nitrogen, and phosphorus from land to ocean - they influence global carbon fluxes by acting as sinks or sources

the global carbon cycle as freshwater

freshwater plays an intermediate roles, processing and altering carbon before it reaches the ocean - major reservoirs include dissolved organic carbon and particulate organic carbon

lakes and reservoirs as regulators of carbon cycling and climate

influence greenhouse gas emissions and retain organic material which prevents carbon export to oceans

what happens in the pipe

carbon is transformed through microbial respiration, sedimentation, and outgassing - nutrients cycle between organic and inorganic forms

C, N, and P through the freshwater pipe

the balance of C:N:P ratios affects productivity and nutrient limitation - deviations from Redfield ratios impact ecosystem function

redfield ratio

CNP : 106:16:1

carbon cycling in freshwater

includes processes such as photosynthesis, respiration, burial and outgassing - CO2 and methane emissions are significant components - about 3000 in and 1000 out via export (31%) and 2000 (64%) out vis gas and 5% is burial

nitrogen cycling in freshwater

involves nitrification, denitrification, ammonification, and nitrogen fixation - microbes play a key role in nitrogen transformations - about 100 in and 30 (44%) out and 40 (40%) out via gas 16% is buried

Phosphorus cycling in freshwater

P is limiting nutrient in many freshwater ecosystems - it cycles between dissolved and particulate forms, often controlled by sediment interactions - about 9 in and about 4 (44%) out via export, and 0 (0%) out via gas and 54% is buried

microbial role in nutrient cycling

microbes mediate many different N-transformations, including exchange with the atmosphere - all microbes use P, but redox transformations are not nearly as relevant - there is essentially no gaseous P (except for PH3)

what drives denitrification

bacteria drive denitrification, converting NO3- and N2 gas - nitrifers oxidize NH4+ to NO3-, linking nitrogen to C and O cycles

amount of burial in P, N and C

P>>N>C - P burial is 10x C and 3x N

what percent are P, N and C exported at

40%

what are gas losses huge for

C and N but not P

what alters the stoichiometry from C-rich terrestrial material to more closely resemble marine plankton

freshwater

what is burial stoichiometry rich in

P

what is loss to atm stoichiometry rich in

Carbon

what element has a great deal of internal production

Carbon - primary production - interplay of C, N and P determines primary production and ecosystem health

what can imbalance of N, C and P lead to

eutrophication or nutrient deficiencies

limited commonly in freshwater

N and P limitation varies by ecosystem; lakes often shift from N to P limitation over time - anthropogenic influences alter natural nutrient availability

physical limiting factors to FW production

temp, light availability and mixing depth influence biological productivity - thermal stratification affects nutrients recycling

nutrient limitation

P limitation is more common due to sediment binding and slow recycling - N limitation can occur in high P environments with denitrification loss

what nutrient have lakes transitioned to being limited in

P limitation due to long-term nutrient inputs - human activities accelerate P cycling through runoff and pollution

what is the is the dominant freshwater paradigm

P-limitation because there is no atmospheric reservoir for P - if a lake is N-limited, it selects for N-fixing cyanobacteria which increase the input of N and the lake will evolve to be P limited

when lakes are more productive (higher chlorophyll levels)

they are more likely to be N deficient - indicates a large biomass of phytoplankton which are primary producers that consume nutrients like N - deplete readily available N in water column leading to a N-deficient state even if overall N levels are relatively high

N2 saturation

lakes in upper midwest are often supersaturated with N2 gas - this is driven by high rates of biological nitrogen fixation and anthropogenic nitrogen inputs

is P or N easier to manage

P because it is very insoluble, while N (especially as nitrate) is quite soluble - managing N requires facilitating denitrification to remove N (lakes, sewage treatment, etc) - we add a LOT of N and P to agricultural landscapes

why are aquatic food webs different than terrestrial food webs

primary producers in aquatic systems are mostly algae and cyanobacteria, whereas terrestrial systems rely on vascular plants - aquatic systems have shorter, more dynamic trophic interactions

what are aquatic food webs dominated by

fast-growing, low biomass primary producers - energy transfer in aquatic systems is more rapid due to the dominance of microbial and planktonic pathways - there are higher trophic efficiency in aquatic ecosystems due to rapid nutrient recycling- algae and phytoplankton have short lifespans but high productivity - rapid turnover leads to efficient nutrient use and strong coupling between trophic levels

what are terrestrial food webs dominated by

slow growing, high biomass primary producers

the predominant form of DIC in most MN freshwaters is

bicarbonate

a 2 µm particle sinks in lakes faster than a 10 µm particle

false

most algae are negatively buoyant

true

carbonate precipitation should occur more prevalently in lakes in the arrowhead than in south-west Minnesota

false

more organic carbon is buried in freshwater ecosystems than in all of the ocean

true

How many Pg of carbon (mostly as CO2) are released by freshwaters to the atmosphere annually?

2-5

Why are the C:P ratios of particles in freshwaters higher and more variable than those in the ocean?

There is greater influence of terrestrial organic matter and residence times are more variable

Schindler's experiments at the Experimental Lake Area in Canada demonstrated that: 

Lakes are limited by P