Tipping points - Lecture 9 (tipping points in lakes)

what are the two main drivers for lakes

• climate (seasonality)

  • what is going on with the catchment (changing veg, deforestation, farming)

how does climate influence lakes

it influences the hydro logical budget which leads to change in the vegetation dynamics which leads to a change in the water chemistry and nutrients

what is eutrophication

the process by which an entire body of water, or parts of it becomes progressively enriched with nutrients, particularly nitrogen and phosphorus

how does nitrogen and phosphorus affect lakes

they act as fertilisers for plant growth

why does lakes being phosphorus limited bad

because if the whole catchment changes (an increase in phosphorus) the it spirals out of balance and become eutrophic

how does phosphorus and oxygen interact

bacteria breaks down dead plants and animals causing phosphorus to be released into the water. When oxygen is present it provides a thin film in the surface sediemtns which prevents phosphorus from rising from deoxygenated sediments. HOWEVER, if the water is deoxygenated phosphate is realsed

how does phosphorus and oxygen cause a positive feedback

increased anoxia promotes more P release which enhances eutrophication

what is happening to lakes as they decrease

they increase in salinity as they get smaller however the salt content remains the same

what is happinging at lake bogoria

it is a flamingo lake where it is getting wetter and wetter - the freshwater level is increasing (it is around 25% bigger than it was in 2013!)

what are the two types of lake tipping points

algal dominated and plant dominated

how to know if a lake has tipped

Observed abrupt shifts from clear water macrophyte states to turbid algal-dominated states. Shifts appear to map on to critical transition theory – nonlinear fold bifurcations and hysteresis. Theory points to early warning signals of tipping points

why is it hard for a lake to go back to what it once was

because there are so many feedbacks operating in it at one point - there is a lag behind the forcing before it can return to its original state

what is happening at lake Erhai, Yunnan

Low eutrophic state, clear water……… ……..after 2002, hypereutrophic state - algal blooms and emergent macrophytes only. there have been two algal blooms in the past

what are slow drivers

trends of temperature rising, rainfall rising, water level rising, nutrient (P) loading from catchment farming and fish yields indicating rising aquaculture in preceding decade. Note the relatively low water level since 1970s.

what are fast drivers

temperature, rainfall, water level driven by seasonal climate extremes and HEP management. Total dissolved P rises but drops a couple years after transition despite continued rise in fertilizer – possible effects of new controls on fish farms and polluted runoff. Submerged macrophytes declining in previous decade

what is alternate state

Phase plots between total phosphorus (TP) and water clarity (SD) and chl-a (phytoplankton proxy) indicate persistent alternative stable states where TP ~0.018-0.03 mg/l and hysteresis.

what is frequency domain analysis

rising variance is a key early warning sign - (less resiliance)

why was there a tipping point at Erhai

• combination of rising fertiliser usage, low water levels and warm dry summers

the gradual changes led to the tipping point - but even through nutrients are now more controlled, system is not recovering

How to measure resilience & structure: Network Skewness

Reconstructions through time show that positive skew reduces with temporal intensification of human impacts in the lake and surrounding catchments, and rises as lakes recover from disturbance

can some lakes be reversable

yes! step change is a thing however, auatic communities may not ever fully recover.

what are lake management options

• reducing nutrients

• biomanipulation