Lec 5 - Lake Trophic State

Sources of contaminants into lakes:

Lake are connected to an catchmment and recieve contaminants and water from surrounding land use.

  • Agriculture

  • Wastewater treatments

  • Urbanisation

  • Storm drains

Basic Lake function:

Main water body:

  • Input from catchment, and output from catchment.

Benthic area:

  • Bottom up processes.

Main 2 concepts: Report

Lake residence time (Turn over time)

Claculation:

\frac{\left(LakeVolumekm^3\right)}{\left(InflowVolumekm^3y^{-1}\right)} = Residence in units of time.

  • Refers to how long an particular water, pass on average remians in the lake.

  • Important to understand, how contaminants will pass through the lake, and how long will this process take from entering to exiting .

Retention - R%

Equation - Need to fill in

  • Understand how much contaminant is entering the lake, and how much of the contamination is staying in the sediment.

  • Sediment acts as an long term sink for contaminnts and other particles.

  • The retention percent can be claculated as the amount of contaminants that are lost and buried in the sediment.

Water balance/Nutirent budgets

  • Estimate the maount of load from an contaminant into the lake, we can measure conatminant in outflow and calculate how much contaminant is leaving the lake, relating to the retention.

Cultural Eutrophication:

  • Biggest challenge for lakes globally.

  • Eutrophication - Change from one trophic state to another (Usually the increse in Nitrogen and Phosphorus, plant nutrients to change the state from an oliogtrophic lake to an Eutrophic lake.

Example:

Oligotrophic lakes - Low nutrients/Low algae growth/High water clarity

Mesotrophic lakes -

Eutrophic lakes - High in nutreints, High algae grwoth/Decreased water clarity

Carlson 1977:

  • The concept of eutrophication started off as an hypothesis, of the agricultural industry uncreasing the amount of nitrogen and phosphorus entering the lakes cauing an growth in algae.

  • They measured phosphorus, chorphyll-A, and water clarity to translate the scale of eutrophication.

NZ Lakes relatonship with nutrients:

Eutrophication in primary production New-Zealand lakes can be limited by Nitrogen and Phosphorus.

  • Phosphorus is a key driver for Eutrophication.

  • Nirogen has an stronger relationship with Chlorophyll-A compared to areas in the Northern temperate zone.

NZ Trophic Level Index (TLI)

Trophic level index that which describes eutrophication in lakes for New-Zealand specific conditions.

  • Using Chorophyll-A as an primary indicator and not water clarity, because NZ lakes are optically diverse, meanining there are natural drivers reducing water clarity.

  • Monthly to quartly smapling

Example:

  • In the south island glacier water, reducing water clarity in glacial lakes.

Strengths:

  • Combining 4 values to get an single value numerically.

  • Easy and cheap to measure

  • National application

  • Indicate type of required intervention

Weaknesses:

  • Can’t pick up subtle changes in nutrient changes.

  • No immediate link to a management response.

  • Not designed as an management tool,

  • Not predictive

  • Blunt tool

Calculations for TLI: Report

Add equations : Data collected will be analysed in data workshop

Deriving an equation for Phorphorus/nitrogen water clarity

Catchment land use and trophic state:

  • Catchmemnts on pastoral land, have high TLI values, compared to lakes with predominantley natural catchment.

Exotic forestry in the North Island delivers a substantial amount fo nutrients to those lakes, more comparable to pastoral cover than natural cover values.

Anthropogenic activities:

Increase in Industry and agricultural activities, there is an increase in atmospheric concentrations of Nitrogen, and every time it rains nitrogen phosphorus id deposited intio lakes.

National objective framework standards for Lake Chlorophyll-a

  • New framework in NZ to measure trophic state.

  • Freshwater management policy.

  • All NZ lakes need to be above the national bottom line, which is between Eutrophic (C) and Supertrophic (D) or worse.

  • 3-5 yrs (Blunt)

Not all lakes are the same!

Shallow lakes:

  • Continuously highly dynamic, due to the effect of surface waves on bottom nutrient cycling (Year round).

  • Exposed to winds, that drive surface waves that can cause bottom sediment resuspension, recycling nutrients.

Example:

  • Lake waahi

Deep lakes:

  • Nutreints will enter lake, get processed mostly in the surface layer (Epiliminiom), taking a much longer time for this nutrinets to reach the bottom making the processing time in the surface layer is much longer compared to shallow lakes.

  • Pronounced seasonal dynamics, becuase the nutrients takes a long time to reach the bottom, chemical and biollogically driven by bottom nutrient cycling.

  • Dark bottom water

Example:

  • Lake Puketirini

Lake response to changes in nutrient loads: Report

Add graph - Response from High to Low

Explain this graph and redraw it.

Add graph - Response from Low to High

Internal loading and delayed recovery

Shallow lakes graph

Fresh hold behaviours

Reduce nutrient loads

Different Lake trophic states:

  • Ootra microtrophic lakes - Pure clean water (Soi

  • Oligotrophic lakes

Calculations for calculating TLi: Important in report

Catchmnet land use affecting lake trophic states:

  • Agriculture

-Low water quality

-Vulnerable to nutrients

-Reduce wtaer quality

Anthropogenic activities driving large scale changes in Lake trophic state:

  • Eutrophic

  • Super trophic

  • Hypertrophic

Geological patterns:

Increase in industry, and agriculture from atmosphere can cause

What do you use TLI for:

  • Easily measure

  • Cheap

  • National application (Used over most lakes)

  • Indicate type of intervention

  • Blunt tool

  • It’s not rythmic, dont pick up subtle changes.

  • Need targetted response, because does not tell you the specific chemical that needs to be intervened.

  • Not used as an mangement tool.

  • Can’t predict TLI directly, need to predict the other underlying factors.

National objective framework standards for Lake Chlorophyll-a

Not all lakes are the same!

Deep lakes:

Pronounced seasonal dynamics

Chemical/Biological driven nutrient cycling

Dark bottom waves

Shallow lakes:

Highly dynamic, continuously so, surface waves hitting bottom, reintroducing nutrients into the water. (Bottom nuttrient cycling)

Shallow lakes - Highly dynamic, often exposed to winds thta case surfave waves, causing the bottom to turn up, increasing the nutrients in the surface. The process time in the surface layer is longer compared to shallow lakes.

Examples:

Lakes Waahi and Puketirini

Lake responses:

Deep lakes:

  • Lagged response - The internal loading continuing, even when the external load is reduced.

  • Reducing nutrient load through catchment management.

Shallow lakes:

  • Reduce nutrient loading, to get back to degraded state (Takes a long time)

  • Push ecosystme to tipping point, adding a little more nutrients.

  • Thresholds are different