Gut-fluorescence

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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Mesozooplankton ingestion (gut fluorescence)
Approach: fluorometric measurement of gut pigment content × gut evacuation rate
Context: in situ, lab
Spatial scale: point sample (net tow)
Temporal scale: hours to days
Units: mg Chl-a m-2 d-1; mg C m-2 d-1
Community captured: selected herbivorous species (> 0.5 mm; typically copepods, euphausiids)
Co-measurements: Chl-a, phaeopigments; gut evacuation rate (k'); C:Chl ratio

Method Overview

The gut fluorescence method estimates ingestion rates of herbivorous mesozooplankton (copepods, euphausiids, and other species > 0.5 mm) from the chlorophyll-a content of their digestive tracts. Freshly caught animals from net tows are immediately frozen or fixed in liquid nitrogen in the dark to halt pigment degradation. The gut contents are homogenized and extracted in acetone; chlorophyll-a and phaeopigment concentrations are quantified fluorometrically before and after acidification. The gut pigment content (G, µg Chl-a animal^-1) is multiplied by the gut evacuation rate constant (k', h^-1), which is determined from separate dark-bottle experiments tracking pigment loss from living animals over time: Ingestion (I) = G × k'. Depth-integration and multiplication by species abundance yields areal ingestion rates.

Scale of measurement

Net tows typically integrate over tens to hundreds of meters of water column, providing depth-averaged estimates. The integration time of the gut content varies with gut transit time, which is temperature dependent (typically 0.5–2 h at in situ temperatures).

Data generated

Per-animal ingestion rates (µg Chl-a animal^-1 d^-1) and community ingestion rates (mg Chl-a m^-2 d^-1). Conversion to carbon requires the C:Chl ratio, which must be measured or estimated for the prey community.

Units & currency

Units are mg Chl-a m^-2 d^-1 or mg C m^-2 d^-1. The currency is Chl-a.

Sample size

Typical catches of 20–200 individuals per species per sampling event.

Limitations

The method assumes that gut evacuation rate (k') and assimilation efficiency are constant for a given species and temperature during the sampling period. The choice of phaeopigment index as a measure of gut fullness is somewhat arbitrary; phaeopigments are degradation products of chlorophyll and may not accurately reflect the true amount of chlorophyll ingested, particularly when animals feed on non-chlorophyll-containing prey. Converting gut pigment to absolute ingestion and then to carbon ingestion introduces multiple conversion factors, each with its own uncertainty.

Example Applications & Protocols

Classic examples

• Mackas, D. & Bohrer, R. (1976) Fluorescence analysis of zooplankton gut contents and an investigation of diel feeding patterns

Common calculations/conversions

• Ingestion rate I (µg Chl-a ind^-1 d^-1) = G × k' × 24; where G is mean gut content (µg Chl-a) and k' is the evacuation rate constant (h^-1).
• Carbon ingestion = I × C:Chl ratio (g C g Chl^-1); typical values 30–100 g:g for marine phytoplankton.