In situ stimulated fluorescence is used to detect the presence of
phytoplankton in water bodies. The intensity of fluorescence is generally
described as 3% of the total absorption of light energy by chlorophyll
containing cells (phytoplankton). The photosynthetic unit (reaction
center) of chlorophyll emits excess light energy in the form of heat,
fluorescence (short time lag after absorption), and/or phosphorescence
(longer lag time after absorption). Fluorescence intensities from depth
profiles of fluorescence are commonly used to represent [chl]. Due to
differences in fluorescence quantum yield (quanta fluoresced per quanta
absorbed) and absorption quantum yield (quanta absorbed per quanta
impingent), chlorophyll concentrations derived from in situ stimulated
fluorescence may be underestimated. Photoinhibition results in reduction
of fluorescence efficiency compared to absorption efficiency. Although
photons are absorbed, excessive light inhibits transfer of electrons from
the light harvesting to the photosynthetic reaction centers. Without
electron transfer, fluorescence does not occur.
Depth profiles of fluorescence intensity measured with a WETstar in situ
fluorometer (WETlabs) were compared to in situ absorption measurements
(ac9, WETlabs). These in situ measurements were compared to discrete
measurements of chlorophyll concentrations determined by in lab stimulated
fluorometry (Turner fluorometer) and absorption by phytoplankton (SLM
Aminco DW-2c spectrophotometer).
Table 1 shows geographical location and time of sampling for East Sound on
August 5. Differences in time and location are not great. Significance of
zenith angle differences was not investigated.
Figure 1 shows the variations in fluorescence intensity (FL Volt), in situ
absorption (a_676), chlorophyll concentration ([chl]), and lab measurements
of absorption by phytoplankton at (a_phi, 676 +/- 5nm). On August 5th,
1998, 3 research vessels (Nugget, Barnes, and Humpback) made separate
measurements of these parameters (Fig 1d-f). In Figure 2a (Nugget),
surface photoinhibition of photosynthesis can be inferred from the inverse
proportionality of a_676 and Fluorescence voltage (Fig. 2c). The Barnes
data (Figure 3c) show further evidence for photoinhibition: absorption
increases at the surface while fluorescence remains constant.
Except when surface photoinhibition occurs, lab measurements of absorption
(a_phi) and chlorophyll concentration ([chl]) follow the same trend as
a_676 and fluorescence intensity. In situ absorption in excess of
fluorescence intensity at depths where photoinhibition is improbable
indicates the presence of a degradation product of chlorophyll,
phaeophytin. Phaeophytin is formed after enzymatic digestion by
zooplankton and absorbs at the same wavelength ranges as chlorophyll.
Phaeophytin concentration may be greater at depth than at the surface
because it sinks as fecal pellets or travels out of the photic zone within
vertically migrating zooplankton.
Changes in slope in salinity and temperature profiles indicate
stratification of different layers (fresh vs. salt water, warm vs. cold
water); constant slopes indicate that the water is well mixed (2-8 b.).
Fluorescence or absorption peaks indicate phytoplankton patches.
Coincidence of salinity, temperature, fluorescence and absorption peaks is
a strong indicator of plankton patches.