L8 - Lake zones and trophic state Flashcards
Thursday 14th March
- Lake Zones dont always occur across all lakes
- Broken down horizontally and vertically
Littoral zone
The depth to which benthic plants can grow (photosynthesis). Benthic vegetation therefore plays an important part in food webs whilst in the pelagic zone it is only really the phytoplankton.
Emergant vegetation
Plants around the edge of the lake
Pelagic zone
- Phytoplankton
- Zooplankton
- Fish
Planktonic processes dominate a lot of discussion of lakes, but they are only a part of the story. Especially in small lakes, and most lakes are small.
Riparian (littoral zone)
Mainly we think about riparian zones in rivers and streams.
- Shade, sediment retention, coarse organic carbon.
- Habitat
Same applies in lakes, habitat is especially important for littoral insects.
These are the types of organisms we associate with lake littoral zones.
Emmergent vegetation important biodiversity habitat.
Really important source of carbon.
Some plant amphibious, essentially rooted in water but leaves always in the air. Stems of emergant plants hollow which allow O2 transportation from atmosphere down to roots which is important as the emergent sediment is organic rich and low in oxygen. Plants would not be able to sustain root function without oxygen.
Eleocharis sphacelata
Can grow as much as 6 metres deep, produces leaves 8 metres tall.
Distribution of these plants depends on their ability to oxygenate their roots.
Values of marginal vegetation
- Habitat for plants and animals (including fish and inverts)
- Buffer to wave action (small waves)
- Nutrient retention
- Intercept groundwater
- Aesthetics
Vegetation-defined zones in the littoral
Complicated in north island.
Koura graze on stonewart and stop them fully growing.
North island has lots of invasive species in the littoral zone which displace natives. (oxygen weeds etc)
12 - 1 - 5 - 8: Typical number of species diversity per zone.
Invasive weeds
Why care about these lake systems?
In big clear water lakes, the littoral zone is so big and water so clear the littoral zone becomes disproportiantly important in terms of carbon flow and food webs dynamics.
Lake Coleridge
Dominated by Characean growing down to 35/40m depth because of water clarity.
It’s so clear because there is almost no phytoplankton occuring in it. All of the food supply is occuring in these communities rather than right out in the pelagic zone.
Characeans form basis of food web.
Almost nothing eats the Characeans but they are an important habitat for epiphytes (single celled algae growing on deep growing plants) and epiphytes support all of these grazers, and all the grazers support native species and Salmonids.
Littoral zone crucial to life in lakes such as Lake Coleridge.
Examples of life in the food web sustained by Epiphytic algae in the littoral zone.
What is it which controlls diversdity and distribution?
Low mound community
Water level and variability and wave action.
Upper limmit
Wave action
Lower limit
Water clarity, competitive interaction between the two - water clarity effects light at depth. High water clarity = deep light penetration. Some plants adapted to grow in poor light conditions but quilworts need lots of light so cannot grow deep without it, tall vascular plants need less light so they can grow deeper. A point is reached where competitive interaction between quilworts and tall vascular plants is set by the amount of light they get which is set by the water clarity.
Natural vs Cultural Eutrophication
Lakes naturally eventually fill up with sediment and become wetlands.
Cultural Eutrophication
- Perhaps the biggest challenge facing lakes globally
- Rapid, does not require filling in, it just involves increased productivity
Lake Taihu China
Carlson one of first people to think about the problem and how to explain it to people. He came up with the trophic lake index which explained the state of the lake easilly.
Eutrophication results previously based on species growing in lake as some species associated with eutrophication and some not.
Carlsons index was based on score between 0-100.
Dominant variability was water clarity as it is something everyone understands and poor water clarity usually means poor water quality.
NZ slightly different to Northen hemisphere where Carlson index works really well.
Correlation between N and P, you will see we have better relationship between N and Ch in NZ than we do in Northern hemisphere. The P is slightly better aswell but N is most important in NZ than Northern hemisphere.
Prediction power of N and P are kind of similar. Where as in the Northern Hemisphere they’re not.
Noel Burns developed New Zealand TLI.
Low number good
High number bad