06kyewalyanga 82W(ds) 2/7/02 8:50 am Page 687 JOURNAL OF PLANKTON RESEARCH VOLUME NUMBER PAGES ‒ Effect of Mesodinium rubrum (= Myrionecta rubra) on the action and absorption spectra of phytoplankton in a coastal marine inlet MARGARETH KYEWALYANGA*, SHUBHA SATHYENDRANATH1,2 AND TREVOR PLATT2 INSTITUTE OF MARINE SCIENCES, UNIVERSITY OF DAR ES SALAAM, PO BOX , ZANZIBAR, TANZANIA, 1DEPARTMENT OF OCEANOGRAPHY, DALHOUSIE UNIVERSITY, HALIFAX, NOVA SCOTIA BH J AND 2BIOLOGICAL OCEANOGRAPHY DIVISION, BEDFORD INSTITUTE OF OCEANOGRAPHY, BOX , DARTMOUTH, NOVA SCOTIA BY A, CANADA *CORRESPONDING AUTHOR: EMAIL: [email protected] This study, carried out at a single station in the Bedford Basin (Nova Scotia, Canada), examined time-dependent changes in the physical and chemical conditions of the waters with associated changes in species composition and some photosynthesis properties of phytoplankton. The sampling period covered late summer months (August and September) and fall months (October to December). Changes in the water conditions were found to influence species composition, which in turn had an effect on the shapes and amplitudes of the action and absorption spectra of phytoplankton, and on the maximum quantum yield of photosynthesis. The most remarkable change was observed in October during a bloom of Mesodinium rubrum, a ciliate harbouring photosynthetic endosymbionts rich in phycobilins. The presence of M. rubrum yielded atypical shapes of the measured photosynthetic action spectra; furthermore, other photosynthesis properties changed significantly. These results demonstrate that the presence of certain pigments in the water column may be associated with a marked shift from what may be considered typical (representative) photosynthesis properties of phytoplankton. INTRODUCTION Shapes of action and absorption spectra have been shown to vary both spatially and temporally. One of the Knowledge of the action and absorption spectra of major causes of variation in action and absorption spectra phytoplankton is important in the estimation of primary is changes in pigment composition, which could imply production using spectrally-resolved photosynthesis–light variation in the phytoplankton population structure models. If ignored, variation in the action spectrum ␣B(␭) (Lewis et al., 1986; Hoepffner and Sathyendranath, 1992; B ␭ and the phytoplankton absorption spectrum ap ( ) (where Lutz et al., 1996; Schofield et al., 1996). Other factors ␭ is the wavelength, and the superscript indicates normal- known to cause variations in the spectra are acclimation ization to phytoplankton biomass B) may cause errors in to light quality or levels (Sakshaug et al., 1991; Johnsen and the computed water-column primary production. For Sakshaug, 1993; Johnsen et al., 1994; Schofield et al., example, a 20% systematic error in ␣B(␭) could result in 1996); intracellular pigment concentration or cell size an error of up to 10% in the calculated water-column (Sathyendranath et al., 1987; Sosik and Mitchell, 1994; primary production (Kyewalyanga et al., 1997). The Stuart et al., 1998); and the presence of photosyntheti- action spectrum ␣B(␭) is proportional to the biomass- cally-inactive pigments (Sosik and Mitchell, 1995; B ␭ specific absorption coefficient of phytoplankton, ap ( ), Lazzara et al., 1996). However, changes in pigment com- with the proportionality factor being equal to the position and/or phytoplankton population structure are ␾ maximum quantum yield of photosynthesis m (Platt and influenced by short-term (e.g. days to weeks) or long-term ␣B ␭ ␾ ␭ ϫ B ␭ Jassby, 1976): ( ) = m( ) ap ( ). That is to say, vari- (seasonal) variations in environmental conditions. ␣B ␭ ␾ ␭ ation in ( ) may be caused by changes in m( ) or in Variations in environmental conditions induce changes B ␭ ap ( ), or in both. in phytoplankton populations. However, their effects on © Oxford University Press 2002 06kyewalyanga 82W(ds) 2/7/02 8:50 am Page 688 JOURNAL OF PLANKTON RESEARCH VOLUME NUMBER PAGES ‒ photosynthetic properties would depend on the type of with a maximum depth of 70 m and a surface area of phytoplankton group(s) present. Phycobiliprotein-con- ~17 km2. The basin is separated from the ocean by a 20 m taining phytoplankton such as cryptomonads and deep sill; the water inside the basin and open-ocean waters cyanobacteria, which have their maximum absorption in move across the sill in response to physical forcing by the green part of the spectrum, have absorption and winds, tides and freshwater runoffs (Platt and Conover, action spectra that differ significantly from what is con- 1971; Platt et al., 1972). sidered to be typical. An example of organisms that show Water samples were collected weekly from four depths unique characteristics is Mesodinium rubrum (= myrionecta (1, 5, 10 and 60 m) at a single station in the middle of the rubra), an autotrophic ciliate that contains endosymbionts Basin from 10 August to 21 December 1994, a total of 20 (cryptomonads) rich in phycoerythrin. Mesodinium rubrum weeks. The water taken from 60 m did not contain any live has been shown to occupy a wide range of environmental phytoplankton, and was therefore excluded from the conditions (Taylor et al., 1971; Crawford, 1989; Satoh and analysis. Data were collected every Wednesday at approxi- Watanabe, 1991; Perriss et al., 1993, 1995; Crawford et al., mately 9.00 a.m. For the determination of absorption 1997). Some of the conditions that have been known to spectra, chlorophyll-a (Chl-a) concentration (by Turner influence the occurrence and abundance of M. rubrum fluorometry), oxygen concentration, pigment composition include increase in temperature and water column (by high performance liquid chromatography, HPLC), stability,which could be caused by heavy precipitation and and concentrations of nutrients (nitrate plus nitrite, sili- run-off (Cloern et al., 1994; Perriss et al., 1995; Crawford cate and phosphate), water was collected at each of the et al., 1997). Another condition is the depletion of three depths using a Niskin bottle. Samples for the action dissolved nitrogen and phosphorus in the photic zone, spectrum and phytoplankton identification were collected since M. rubrum cells can migrate vertically to exploit the only at 5 m. Profiles of temperature, in situ fluorescence (as nutrient pool below the pycnocline (Cloern et al., 1994), a proxy for Chl-a), salinity and photosynthetically avail- and thus out-compete non-motile phytoplankton (such as able radiation (PAR) were determined using a CTD diatoms). probe. Meteorological observations of wind speed and Apart from causing red tides (Barber et al., 1969; Taylor direction, air temperature, rainfall and hours of bright et al., 1971; Crawford, 1989; Crawford et al., 1997) the sunshine were available from Shearwater Airport as daily occurrence of M. rubrum blooms also affects some proper- averages and were obtained from Atmospheric Environ- ties such as the concentration of the main algal pigment, ment Service, Bedford, Nova Scotia. Chlorophyll-a, and the rate of nutrient uptake (Wilkerson The photosynthetic action spectra were determined using and Grunseich, 1990) and primary production (Smith and a spectral incubator, as explained in Kyewalyanga et al. (Kye- Barber, 1979; Laybourn-Parry and Perriss, 1995). walyanga et al., 1997). Water samples were incubated for 3 However, the effect of the presence of M. rubrum on h, under a series of 12 narrow-band (10 nm bandwidth) photosynthesis properties used as inputs in primary pro- irradiances in the range from 400 to 700 nm. At each band, duction models, such as the phytoplankton action spec- 16 bottles (Corning, 70 ml polystyrene culture flasks) were trum, the absorption spectrum and quantum yield, has yet inoculated with 14C and incubated at different light intensi- to be investigated. ties. Another bottle, similarly inoculated, was incubated in In the present study, we examine the effect of changing the dark. To produce the narrow-band light, interference environmental factors and phytoplankton population colour filters (Corion) were used. After incubation, filtration structure on the shapes of action and absorption spectra and fuming to remove unincorporated 14C, counting was of phytoplankton, over a period of five months, in a done using a liquid scintillation counter, and the amount of coastal inlet. In particular, we report on the impact of a 14C fixed was calculated according to Strickland and Mesodinium bloom on the shapes of absorption and action Parsons (Strickland and Parsons, 1972). spectra. The work was carried out at a single station in the Absorption by total particulate materials was deter- Bedford Basin (Nova Scotia, Canada) during late summer mined by using the filter technique (Kishino et al., 1985). to fall 1994. Week-to-week variations in the action spectra, Correction for the pathlength amplification (␤ factor) was absorption spectra, quantum yield of carbon fixation, made using the method of Hoepffner and Sathyendranath pigment (species) composition and hydrographic con- (Hoepffner and Sathyendranath, 1992), and detrital ditions, are explored. absorption was estimated using the approach of Hoepffner and Sathyendranath (Hoepffner and Sathyendranath, 1993), which assumes an exponential function for the METHOD shape of the detrital absorption spectrum. Fourth deriva- The study was carried out in the Bedford Basin, Nova tive of the absorption spectra (Butler and Hopkins, 1970; Scotia, Canada. Bedford Basin is a small marine inlet, Owens et al., 1987; Bidigare