The trouble with Chlorophyll as a Development Objective

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The trouble with Chlorophyll as a development objective

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

July 26, 2006

Overview

The problems w.r.t. chlorophyll data can be summarized as follows:

the collection and submission of chlorophyll samples require precautions that are complex compared to other trophic status parameters

changes in analytical methods may disrupt long-term chlorophyll data sets

significant seasonal and inter-annual variation in chlorophyll requires the collection of large numbers of samples over many years

many different chlorophyll pigments are commonly measured, i.e.: Chl a, b, c, chl a corrected, etc., concentrations of these pigments may not correspond to actual phytoplankton cell densities.

Introduction

Resource managers and researchers from many agencies commonly use Chlorophyll as a trophic status indicator. Although variation in Chlorophyll concentration tends to be the most evident consequence of changes in trophic status, there are problems involved with using this test as a basis for either setting trophic status objectives or detecting long-term change. Development objectives for individual lakes that are based on Chlorophyll will therefore be difficult to assess since it will be impossible to tell the difference between the actual surpassing of objectives and simple variation based on the collection of too few samples. These problems tend to increase in severity with increasing trophic status such that the situations that require the most attention, i.e.: more enriched systems, also tend to require the most samples to describe accurately.

This is not to say that chlorophyll data should not be collected. A great deal of useful data exists that show the effects of phosphorus load reductions, zebra mussels, etc. on chlorophyll concentrations. These are generally based on large data sets that are not plagued by seasonal or inter-annual variation.

Since virtually none of the same problems apply to the collection of total phosphorus data, it is probably better to use total phosphorus (TP) as an indicator of trophic potential in situations where nominal data sets are being collected.

If information about the phytoplankton community must be collected, managers should consider collecting seasonal composite phytoplankton enumeration samples. Generally, weekly, bi-weekly or monthly phytoplankton samples are collected and fixed with Lugols fixative. These may be combined at an enumeration lab and counted to provide seasonal mean, phytoplankton cell densities. These number will relate better to trophic status than will chlorophyll estimates.

Data collection

Chlorophyll samples must be collected into opaque bottles and immediately fixed with magnesium carbonate (MgCO₃ ensures that the sample remains "basic" to avoid conversion of primary pigments to phaeopigments under acidic conditions). They must then be kept cool and filtered as soon as possible. The filtrate must be frozen and transported to the lab without being allowed to thaw. This makes the remote collection of samples difficult or impossible such that, from the onset, chlorophyll data can present uncertainties if the samples have not been collected under strictly controlled conditions.

Chlorophyll samples are often collected as euphotic zone composites. This zone is sometimes well mixed since much of this layer is composed of the epilimnion. However algal cells will often stratify dramatically below the epilimnion and this can occur even in mixed layers. This means that chlorophyll concentrations based on euphotic zone composite samples may vary based simply on the physical collection methods i.e.: how the water is combined in proportion from given depths. This is very relevant in situations where the depth of the euphotic zone relative to the thermocline changes over time.

Seasonal and Inter-Annual variation

The largest problem with the interpretation of chlorophyll data is associated with seasonal and inter-annual variation. Chlorophyll concentrations vary significantly on a seasonal basis within lakes and often show different seasonal patterns between lakes. In addition there is a great number of long-term, or between-year variation in the ice-free means for individual lakes. This makes it necessary to collect numerous samples each year to derive ice-free means that are close to the actual value, and many years of this type of data are required to estimate the long-term mean.

Cell Density vs. Pigment concentration

The whole picture is further complicated by the fact that chlorophyll concentrations are not always tied to phytoplankton cell densities. The actual concentration of chlorophyll in algal cells is determined by incident radiation, species composition, nutrient supply and certain aspects of algal physiology. These determinants have a seasonal component such that the correspondence between chlorophyll a and algal cell densities is not constant.

These relationships can further be specific to different chlorophyll pigments. In most cases chlorophyll a or a version of chlorophyll a corrected for phaeopigments is used to represent the phytoplankton community. Sometimes chl b or chl c are quoted but often the relationship between the concentrations of specific pigments and the concentrations of algal cells in greatest abundance do not contain pigments that are being measured. Also, algal communities are changing seasonally back and forth between those that contain the investigator's pigment of choice and those that do not.

........... per OECD (Vollenweider and Kerekes, 1982), "One serious weakness of the use of Chlorophyll a to represent algal biomass is the great variability of cellular chlorophyll content depending on algal species. Chlorophyll content of algae can range over from 0.1 to 9.7 per cent of fresh algal weight. Radiation intensity and nutrient availability, particularly nitrogen, appear to be major factors affecting the chlorophyll content of algal cells. Although chlorophyll a reflects algal biomass quite well as a whole, a great variability in individual cases can be expected, either seasonally or on an annual basis due to a species composition, light conditions and nutrient availability".

References

Vollenweider and Kerekes, 1982. Eutrophication of Waters, Monitoring, Assessment and Control. (OECD) Organisation for Economic Co-Operation and Development. 155p.

Ontario Ministry of Environment and Energy. Aug 1996. The Trouble with Chlorophyll: Cautions Regarding the Collection and Use of Chlorophyll Data. STB Tech. Bull. No. AqSS-10.

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