Water Pollution - Periphyton Growth Potential

Periphyton Growth Potential estimates the risk of periphyton (algae and microorganisms) growth in a river basin based on high nitrogen and phosphorus concentrations. Excessive growth can cause ecological imbalances.

Overview

Data Type	Combination of monitoring data and modelling approach
Source	McDowell et al (2020)
Spatial Resolution	Spatially explicit to river basins (HydroBASINS level 6)
Temporal Resolution	Based on monitoring data between 1990 and 2016. Official updates to be confirmed.
Interpretation	The periphyton growth potential is estimated based on the concentration and ratio of nitrogen and phosphorous in local waters. The combination of concentration and ratio values are categorized into a 0-10 score based on thresholds established by scientific literature.
Concern Level	Very Low = Score 0 - 2 Low = Score 2 - 4 Moderate = Score 4 - 6 High = Score 6 - 8 Very High = 8 - 10
Framework Relevance	Periphyton Growth Potential is relevant under CSRD (ESRS E2 and ESRS E3) as well as under TNFD and SBTN.

Detailed Specifications

Explanation

Periphyton refers to a community of microorganisms, including algae, that grows on the beds of streams and rivers. Periphyton growth is a response to elevated nutrient levels associated with eutrophication. Excessive periphyton growth can contribute to the negative effects of eutrophication as the dense biomass may lead to algal blooms, oxygen depletion, and changes in water quality. The Periphyton Growth Potential (PGP) indicator estimates the potential of undesirable periphyton growth in a given river basin. The indicator is based on several datasets and a global model of dissolved and total nitrogen (N) and phosphorus (P) concentrations. The amount and ratios of N and P are used to determine the potential of periphyton growth as well as which nutrient, nitrogen or phosphorus, limits periphyton growth during the growing season (when considered in terms of mass, if the N:P concentration ratio is >7:1, it suggests phosphorus limits growth, while if the ratio is <7:1, nitrogen limits growth).

Transformation & Interpretation

The Periphyton Growth Potential (PGP) indicator provides the estimated raw values of total nitrogen (N) and phosphorous (P) concentration. The indicator is translated into a 0-10 scale based on the N:P ratio (to identify the growth limiting nutrient) and the concentration value of the limiting nutrient (mg/L). The following thresholds are used to convert ratios and raw values into scores and concern levels:

For N-limited periphyton growth (N:P ratio < 7)

Raw Value (mg/l)	Score	Concern Level
N ≤ 0.4	0 - 2	Very Low
0.4 < N ≤ 0.8	2 - 4	Low
0.8 < N ≤ 1.2	4 - 6	Moderate
1.2 < N ≤ 1.6	6 - 8	High
N > 1.6	8 - 10	Very High

For P-limited periphyton growth (N:P ratio ≥ 7)

Raw Value (mg/l)	Score	Concern Level
P ≤ 0.023	0 - 2	Very Low
0.023 < P ≤ 0.046	2 - 4	Low
0.046 < P ≤ 0.1	4 - 6	Moderate
0.1 < P ≤ 0.15	6 - 8	High
P > 0.15	8 - 10	Very High

Source

The Periphyton Growth Potential indicator provided as part of SBTN’s Water Pollution layer is sourced sourced from Camargo et al. (2023). The underlying data stems from McDowell et al (2020).

Date of publication

2023 (2020)

Confidence & Accuracy

Medium (best available at global scale). The Periphyton Growth Potential indicator is based on historic data and a global model. The model is expected to be valid for large streams and rivers, might however fail to accurately estimate current periphyton growth potential at the local scale. Nevertheless, the indicator is science-based, peer-reviewed and explicitly required for science-based target setting under SBTN.

Relevance for CSRD

CSRD requires companies to identify operational sites situated in areas of water risk, incl. areas of high-water stress. The Periphyton Growth Potential indicator is relevant under:

ESRS Article	Disclosure or application requirement
ESRS E2 AR 23 (c)	percentage of emissions of pollutants to water and soil occurring in areas at water risk, including areas of high-water stress
ESRS E3 DR 12 (c)	commitment to reduce material water consumption in areas at water risk
ESRS E3 DR 19	actions and resources in relation to areas at water risk, including areas of high-water stress
ESRS E3 DR 23 (a)	impacts, risks and opportunities related to areas at water risk, including improvement of the water quality
ESRS E3 DR 23 (c)	reduction targets for water consumption, in areas at water risk, including areas of high water-stress
ESRS E3 DR 28 (b)	total water consumption in m3 in areas at water risk, including areas of high-water stress
ESRS E3 AR 13 (b)	risks including water quantity (water scarcity, water stress), water quality (...)

CSRD requires companies to consider and describe the data used in their analysis (ESRS E4 AR 27). For convenience, here a description of for the relevant criteria:

ESRS Data Criteria	Comment
(a) methodology, reason for selection, assumptions, limitations & uncertainties	Methodology: Periphyton Growth Potential is estimated based on a combination of monitoring data and a modelling approach. Reason for selection: The indicator has been selected to assess the level of water pollution. The selected metric is science-based, peer-reviewed, widely used and specifically endorsed by SBTN guidelines. Assumptions: The indicator is model based and incorporates several assumptions detailed in McDowell et al (2020). Limitations & uncertainties: The Periphyton Growth Potential indicator is based on historic data and a global model. The model is expected to be valid for large streams and rivers, might however fail to accurately estimate current periphyton growth potential at the local scale.
(b) scope of metric	The Periphyton Growth Potential indicator is applied at site level.
(c) biodiversity component of metric	The Periphyton Growth Potential is an ecosystem-level nature indicator. It assesses ecosystem structure by providing an indicator for the chemical state of the ecosystem.
(d) covered geographies	The Periphyton Growth Potential indicator has global coverage.
(e) integration of ecological thresholds	The Periphyton Growth Potential indicator relates to the planetary boundaries of freshwater change and biochemical flows.
(f) frequency of monitoring, baseline condition and period	The Periphyton Growth Potential indicator is based on monitoring data between 1990 and 2016. Official updates of the indicator remain to be confirmed.
(g) data type (primary, secondary, modelled, expert judgement	The Periphyton Growth Potential indicator relies on modelled data.

Relevance for TNFD

The Periphyton Growth Potential indicator is a suitable indicator to estimate water availability under the criteria of Physical Water Risk within TNFD’s LOCATE step.

Relevance for SBTN

The Periphyton Growth Potential indicator represents one out of three water pollution indicators explicitly required to use as a pressure-sensitive State of Nature indicator in SBTN Step1: Assess and Step 2: Interpret & Prioritize.

SBTN Data & Tool Criteria	Evaluation	Comment
Relevance	✅	PGP contributes to the pressure-sensitive state of nature metric water pollution required by SBTN methodology.
Representative	✅	PGP data represents a close fit with the SBTN characterization of the water pollution issue and is appropriate to be applied under Step 1 & 2.
Spatial and Temporal Resolution	✅	The PGP indicator is provided at the spatial resolution of level 6 of Hydrological sub-basin (HydroBASINS 6). The PGP indicator is based on monitoring data between 1990 and 2016.
Stability and Preservation	✅	PGP data sets are likely to be maintained and preserved long-term as they are provided by SBTN.
Accessibility	✅	The PGP data is readily accessible online.
Interpretability	✅	The PGP indicator is sufficiently interpretable and generates an appropriate input for the SBTN methodology.
Coverage	✅	PGP has global coverage.
Authoritative and Accurate	✅	PGP data is science-based, peer-reviewed and explicitly required by SBTN.