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Land and lake interconnectedness
On a day-to-day basis, most of my time as WTT Research & Conservation Officer is devoted to river habitats, but in my academic role at the Lancaster Environment Centre, lakes are a long-standing focus of my aquatic ecology research. A Natural Environment Research Council grant allowed a colleague from Cambridge and I to convene a workshop with scientists from Canada, the USA and Sweden, with common interests in how lake food webs may be fuelled by subsidies from the land. The output from that meeting is a recently published synthesis based upon data from ~150 northern lakes. Most of the planet's freshwater lakes and rivers that we associate with various ecosystem services, like fisheries and water supply, are found in the northern hemisphere, a region that is changing rapidly in response to human activity intertwined with shifting climatic trends.
The classical view of lake food webs is that of algae produced via photosynthesis forming the food base for zooplankton (such as water fleas or Daphnia) that is then munched by fish. We do not contest this because algae are typically a very high quality diet, and many lakes contain plentiful algae. However, in lakes that do not contain adequate supplies of such a resource, or during winter when it is less available or completely unavailable, then organic matter from the land, derived from the breakdown of terrestrial plants, can still be used via intermediary bacteria. And this situation is more common than you might imagine.
Our team used stable isotope techniques as natural ‘tracers’ of diet and analysed over 550 zooplankton samples from lakes across a range of conditions from the boreal zone to the sub-tropics. In half of the zooplankton samples, these tiny animals comprised at least 42% terrestrially-derived material. In some instances, terrestrial food sources accounted for up to 83% of total zooplankton biomass.
By concurrently investigating how physical characteristics of the land draining into lakes influenced the observed terrestrial subsidies in the lake food webs, we could identify certain key determinants. Differences in tree cover, vegetation density, soil carbon concentrations, lake area, and lake perimeter can all potentially impact upon the amount of terrestrial matter available to aquatic life.
These widespread findings further highlight the interconnectedness of ecosystems. But they also convey an important message for those tasked with conserving and managing freshwater resources. We must understand how landscape changes, such as forest clearing or reinstatement, or agricultural practices, impact upon lakes that they adjoin or are linked to via river networks.
What does this mean for trout? Well, trout are renowned for their ability to maintain higher populations than we might expect for a given stretch of river or area of lake. That is because they can and do readily tap into food resources from ‘outside’ of the waterbody, i.e. daddy long-legs or heather flies, worms, even cicadas, mice and lemmings. However, if some of their ‘aquatic’ food is already heavily subsidised by terrestrial matter, then it is even more important for us to consider sensitive land management beyond a riparian buffer strip.
The full article for those that are brave, is available open access here: http://advances.sciencemag.org/content/3/3/e1601765