Overview
Understory plant assemblages are important sources of primary production in both
terrestrial and marine environments, and their biomass dynamics can be greatly
influenced by their overstory counterparts. Studies of how interactions between canopy
and understory species influence the productivity of the entire community are rare,
particularly in marine systems where measuring primary production of diverse
assemblages of macroalgae is logistically challenging. To overcome these challenges,
we developed a simple model of primary production for understory macroalgae that
relates species-specific light use relationships measured in the laboratory to biomass
and light levels measured in nature (Miller et al. 2012). Here we describe the methods
(and present the corresponding data) that we used to measure respiration,
photosynthesis at non-saturating irradiance (α) and photosynthesise at saturating
irradiance (Pmax) of 23 species of understory macroalgae common
to kelp forests off Santa Barbara, CA.
Laboratory-measured rates of photosynthetis
Twenty-three of the most common species of macroalgae were collected from reefs
near Santa Barbara at a depth of 5–9 m and kept in an indoor aquarium with running
seawater at ambient temperature for no longer than 2 days before photosynthesis versus
irradiance (P vs. E) measurements were made. We used whole thalli in the incubation
experiments to incorporate the effects of plant morphology and self-shading into
production measurements. Each algal specimen was cleaned of all epiphytes prior to
incubation. The holdfast and most of the stipe of the stipitate kelps
Pterygophora californica, Egregia menziesii, and
Laminaria farlowii were removed so the specimens could fit into
the incubation tanks; most of the photosynthetic tissue in these species is in their
blades. Juveniles approximately 30 cm in height were measured for the giant kelp,
Macrocystis pyrifera.
Photosynthesis (P) versus Irradiance € relationships were obtained for each
species by anchoring a specimen with modeling clay in a natural upright position to
the bottom of a sealed acrylic tank (volume 35 L). The tank was submerged in a bath of
running seawater. Tanks were equipped with a submersible aquarium pump (Rio model 50;
262 L h-1) to provide circulation, and an optical probe
(D-Opto; ENVCO) that measured dissolved oxygen at a frequency of once per minute.
Specimens were incubated in the dark for 20 min to measure respiration rate. Tank
seawater was then sparged with nitrogen gas (N2) to lower initial oxygen
concentrations. The nitrogen sparging had no detectable effects on seawater pH.
Irradiance was provided by two 500-W halogen lamps fixed 30 cm above the tanks.
Plastic mesh screens (9 in total) were sequentially removed from the incubation tank
lid at 20-min intervals, creating incubation irradiances of 10 (all screens present),
19, 36, 60, 103, 178, 198, 344, 392, and 700 µmol photons
m-2 s -1, which spanned the
range of irradiances measured in the field. The wet mass and volume of each specimen
were measured following the completion of incubations. Wet samples were dried for at
least 72 h at 60o C and re-weighed to obtain dry mass. Tank
volumes were corrected for the volume displaced by algae, clay, pump, and oxygen
probe. Oxygen evolution rates for each light level and for dark incubations were
calculated by fitting a linear regression to the measured change in oxygen
concentration over incubation time. The regression equation was used to calculate
hourly rates of oxygen evolution per gram of dry photosynthetic tissue. Oxygen
evolution rates were converted to carbon using a photosynthetic quotient of 1. The
initial slope of the curve, α, was calculated by fitting a linear regression to the
change in production rate over a range of non-saturating irradiance values (1–150 µmol
m-2 s -1) for each taxon
(Jassby and Platt 1976). Pmax was estimated individually for
each taxon by fitting the data to the equation presented below using a least squares
non-linear fitting procedure (SAS ver. 9.1.3, PROC-NLIN; SAS Institute, Cary, NC).
We used the methods of Miller et al. (2012) to measure photosynthesis,
irradiance, and respiration by 22 of the most common macroalgal taxa observed at
long-term study sites surveyed by the SBC LTER . Additionally, we measured P versus E
for the reproductive fronds of Cystoseira osmundaceae, which can
exhibit seasonally high biomass. We incubated whole thalli (minus the woody stipe and
holdfast in the case of the kelp Pterygophora californica) in
clear acrylic tanks and measured oxygen evolution at nine levels of irradiance (19,
36, 60, 103, 178, 198, 344, 392, and 700 μmol m-2
sec-1: n = 10 to 20 whole thalli per taxon) that
encompassed the 95th percentile of daylight values recorded
on the bottom at our sites during the period of study. The initial slope of the
relationship between photosynthesis and irradiance at non-saturating irradiance (α)
was determined using linear regression of non-saturating irradiance values for each
taxon (Jassby and Platt 1976). Photosynthesis at saturating irradiance
(Pmax) was estimated for each thallus by fitting the
hyperbolic tangent function (Jassby and Platt 1976) using SAS (SAS Institute Inc.,
North Carolina version 9.1.3). Estimates of Pmax and α were
averaged across replicate thalli to obtain mean estimates for each species or
taxonomic group. Units of oxygen were converted to carbon using a photosynthetic
quotient of 1.0 (following Rosenberg et al. 1995) and respiration and production rates
were standardized to the dry mass of photosynthetic tissue.
References
Jassby A. D., Platt T. 1976. Mathematical formulation of the relationship between
photosynthesis and light for phytoplankton. Limnol Oceanogr 21:540–547
Miller, R. J., S. Harrer and D. C. Reed. 2012. Addition of species abundance and
performance predicts community primary production of macroalgae. Oecologia, 168:
797-806
Rosenberg G. D., Littler S., Littler M. M. and Oliveira E. C. 1995. Primary
production and photosynthetic quotients of seaweeds from Sao Paulo state. Brazil Bot
Mar 38:369–377.
