What is the Biological Condition Gradient (BCG)?

The BCG was initially developed as an approach to assess the biological condition (or “health”) of a freshwater body relative to expectations of “having a composition and diversity comparable to that of the natural habitat” (see Bradley et al. 2020 and Santavy et al. 2012). It is therefore a useful tool to describe how biological conditions change incrementally along a gradient of increasing anthropogenic stress.

Figure 1 Diagram for interpreting the BCG model (EPA 2016)

Currently, the BCG serves as a scientific model that can be fed information about different components of coral reef sites (e.g. reef fish assemblages or benthic species) and uses that information to deliver a value of the relative health of those ecosystems (figure 1). The fish and benthic BCG models can also be combined for a more robust interpretation of biological condition since these assemblages may respond differently to stressors (Bradley et al. 2020).

If the fish and benthic BCGs are additionally coupled with water quality data, the BGC provides a statistically stronger framework to understand the state of coral reef sites. This information can then used to inform management decisions like identifying which water bodies containing coral reefs should be prioritized, protected and/or restored (Bradley et al. 2020).

CCRI’s role regarding the BCG

In accordance with the guidance of the U.S. Environmental Protection Agency (EPA 2002, 2013) and in support of the U.S. Clean Water Act and biocriteria, CCRI and DNER developed a two-year monitoring program for seawater chemistry, benthic communities, and fish assemblages. The data from this project will be used to feed the BCG model in order to assess the biological condition at the study sites; helping us improve management decisions regarding coral reef habitats and seawater quality.

This project is currently administered and conducted by the Caribbean Coral Reef Institute (CCRI) from University of Puerto Rico at Mayaguez (UPRM). The funding provided has allowed us to build the first seawater quality monitoring laboratory from a public institution in Puerto Rico and is located at UPRM’s Isla Magueyes Marine Laboratory in La Parguera, Lajas P.R. The Water Quality Laboratory became fully functional on May 2023.

We are also collaborating to create a “BCG shiny app”, which would allow us to upload data on corals, reef fish, and water quality parameters to calculate the BCG and create a map that showcases the BCG results for each location. The application is publicly available but is currently working only with coral assemblage data collected using Line-Point-Intercept (LPI) and benthic demographic (DEMO) methods.

For more details, access our Quality Assurance Project Plan (QAPP) or Standard Operating Procedures (SOPs).

Project goals

In 1999 the DNER established the Puerto Rico Coral Reef Monitoring Program (PRCRMP) for shallow coral reefs areas (less than or equal to 30 meters) with the following objectives:

For more details, access our Quality Assurance Project Plan (QAPP)
  1. Know the conditions of coral reef species with ecological and economic importance. 
  2. Identify trends in reef communities in response to environmental and human pressures. 
  3. Determine the most effective management strategies for reef protection.

This project aims to:

  1. Quarterly, for two consecutive years, measure 16 water quality parameters in 42 PRCRMP sites (figure 2),
  2. calculate the BCG level of seawater chemistry, reef fish assemblages, and benthic assemblages at those sites
  3. statistically assess stressor-response relationships and,
  4. use this data to support management in coral reef sites.

Study sites

Figure 2 Map of the 42 PRCRMP sites where monitoring of water quality parameters, fish and coral assemblages will occur quarterly from May 2023 – May 2025. Numbers next to the site code in the legend correspond to approximate depth in meters. For more details, access our Quality Assurance Project Plan (QAPP)

Water quality parameters to be studied

Table 1. Summary table of parameters, certified method (SM = Standard Method, EPA = United States Environmental Protection Agency), depth of measurement (surface or bottom), maximum holding time, and established accuracy and precision. Click on any parameter to access a PDF document of our Standard Operating Procedures (SOP).

ParameterMethodDepth of MeasurementHolding Time (hr)AccuracyPrecision
Temperature (°C)SM 2550 BS+BN/A± 0.1<20%
Salinity (psu)SM 2520 BS+BN/A± 0.50<20%
DO (mg/L)SM 4500-O HS+BN/A± 0.2<20%
pHEPA 150.2S+BN/A± 0.002<10%
Secchi Depth (m)Secchi DiskNAN/A± 20%<20%
BOD (mg/L)SM 5210 BS+B48± 0.2<20%
TA (µmol/kg)Dickson et al., 2007S+B672± 3<20%
DIC (µmol/kg)Dickson et al., 2007S+B672± 3<20%
Chlorophyll-a (µg/L)SM 10200 HB24N/A<20%
Enterococcus (MPN)SM 9230 B/C/DB< 6N/A<60%
Turbidity (NTU)EPA 180.1B48± 20%<20%
Settleable Solids (mg/L)SM 2540-FB48±20%<20%
TSS (mg/L)SM 2540-DB48± 5<20%
TP (mg/L)EPA 365.4B672± 0.05<20%
TKN (mg/L)EPA 351.21B672± 0.5<20%
Nitrate + Nitrite (mg/L)EPA 353.2B672± 0.05<20%
For more details, access our Quality Assurance Project Plan (QAPP)

Meet our team!

UPRM – CCRI – Department of Marine Sciences

Department of Natural and Environmental Resources (DNER)

  • Eng. Angel Melendez Aguilar
  • Wanda Garcia Hernandez – WQA Sub-Manager
  • Annette Feliberty Ruiz
  • Angela Garcia Fernandez – Project Manager
  • Maria Vega-Rodriguez, Ph.D. – Project Manager
  • Frances M. Segarra Román – Quality Assurance Manager

U.S. Environmental Protection Agency (EPA)

  • Izabela Wojtenko
  • Carol Lynes – USEPA Quality Assurance Officer

Subcontracted personnel