Coarse, shallow and wooden… but useful nonetheless

Posted on June 19, 2017


Coarse woody debris (CWD) derived from shoreline trees provides an important, spatially complex habitat and a food source for lake-dwelling organisms. Here are some interesting facts about lake CWD, taken from a recent review paper:

  • In the absence of the physical action of flowing water, CWD breaks down more slowly in lakes than in streams. Small twigs decay in a few years but large logs in cold lakes can take up to around 400 years to decompose. Conifers decay more slowly than deciduous trees. In one study, the mean time from the assimilation of carbon in a living tree to its loss in the littoral waters of a lake was estimated at 362-443 years. This sequestration period is around 9-80 times longer than that typical of woody debris in exclusively terrestrial habitats because the decomposition of wood depends on oxygen, which is less concentrated in water than air.

 

  • On a per area basis, the production of algae living on the surface of submerged CWD is 5-10 times lower than that on the littoral sediments of lakes, probably because algae can obtain nutrients (nitrogen and phosphorus) from sediments but not from wood. In contrast, because of the opportunities for shelter provided by complex CWD habitats, the productivity of invertebrates is greater on CWD than on sediments. And further up the food chain, it’s clear from a number of studies that CWD provides important protective, feeding and spawning habitats for fish.

 

  • By reducing stocks of CWD in lakes, human activities such as deforestation, shoreline development and the removal of CWD for recreational reasons can lead to declines in the abundance and diversity of freshwater species. Profound ecological changes were recorded in an experimental study in which 70% of littoral wood was removed from a small nutrient-poor lake. Less than two years later, there had been a significant decrease in the resident population of yellow perch. As a result, predatory largemouth bass switched prey – to smaller fish and invertebrates – which disrupted the food web of the entire lake.

 

Reference:   Czarnecka, M. 2016.  Coarse woody debris in temperate littoral zones: implications for biodiversity, food webs and lake management.  Hydrobiologia 767, 13–25.  https://link.springer.com/article/10.1007/s10750-015-2502-z

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