Interstate Commission on the Potomac River Basin

Life in a Drop of Water: Phytoplankton and Zooplankton

Plankton in water.Plankton are free-floating microscopic algae (phyto-) and animals (zoo-). They are a principal source of food in open waters such as lakes, large rivers, and estuaries where they directly or indirectly support much of the upper levels of the food web.

Other ICPRB plankton projects and reports

Phytoplankton Nutrient Limitation
Water Quality Binning Method
Larval Striped Bass Habitat Index
Chlorophyll Criteria

Phytoplankton Index of Biotic Integrity (PIBI)

Led by ICPRB staff, researchers developed a phytoplankton index of biotic integrity (PIBI) for the Bay in 2005/6 from long-term spring and summer monitoring data and re-validated it with new data in 2013. A water quality binning method combined different levels of water clarity (Secchi depth), dissolved inorganic nitrogen (DIN), and ortho-phosphate (PO4) to create distinct water quality categories (bins) and establish a plankton disturbance spectrum. High quality, “reference” conditions have adequate light for normal plant growth and concentrations of nitrogen and phosphorus low enough to limit algal bloom formation. These are the water quality conditions desired in a restored Chesapeake Bay.

Community-level phytoplankton metrics (e.g., chlorophyll, diatom biomass, dinoflagellate biomass) that most strongly discriminated between reference and degraded conditions were identified, scored, and combined to create the index. The classification efficiencies of the index ranged between 70% and 90% depending on salinity zone and season. The index characterizes balanced, “healthy”, desirable phytoplankton populations in different seasons and salinity zones and is used by resource managers to track progress in Bay restoration.

These publications and reports describe the index development and some preliminary and follow-up studies in more detail.

Zooplankton Indicators

Chesapeake Bay researchers have considered developing zooplankton indicators since 1993 (ICPRB Report 93-2, ICPRB Report 94-4). In 2006, a zooplankton index of biotic integrity was developed for summer polyhaline waters of the Chesapeake Bay with data from long-term monitoring data and the water quality categories used to develop the PIBI. Nine zooplankton community metrics showed strong discrimination efficiencies and were selected for the index. They included the Simpson diversity index and abundances of barnacle larvae, rotifers, cladocerans, copepods, total mesozooplankton, and predators. The index correctly classified about 94% of the degraded samples and about 82% of the reference samples for an overall classification efficiency of 88%.

  • Carpenter, K. E., J. M Johnson, and C. Buchanan. An index of biotic integrity based on the summer polyhaline zooplankton community of the Chesapeake Bay. 2006. Mar. Env. Res. 62(3):165-180.

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