Overview
Understory macroalgal assemblages are important sources of primary
production in marine environments. 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 that we used to measure
respiration, photosynthesis at non-saturating irradiance (α) and
photosynthesis at saturating irradiance
(Pmax) of 22 species of understory macroalgae
and juvenile Macrocystis pyrifera, which are
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 two 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 (E) 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 minutes 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 23 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
