Data Set (knb-lter-sbc.55.1)

SBC LTER: Reef: Understory and phytoplankton production at cleared and control plots, Mohawk Reef, 2007-2008

  Summary and Data Links People and Organizations Temporal, Geographic and Taxonomic Coverage Methods and Protocols  

These methods, instrumentation and/or protocols apply to all data in this dataset:

Protocols and/or Procedures

More information can be found in: Miller, R. J., D. C. Reed and M. A. Brzezinski. 2011. Partitioning of primary production among giant kelp (Macrocystis pyrifera), understory macroalgae, and phytoplankton on a temperate reef. Limnology and Oceanography, 56: 119-132


Study site

This study was done at Mohawk Reef off Santa Barbara, California. The surface canopy of giant kelp was patchy at the beginning of the study and a lush assemblage of understory kelps (Pterygophora californica and Laminaria farlowii), and red algae (principally Chondracanthus corymbiferus and Rhodymenia californica) occupied the bottom underneath the gaps in the Macrocystis canopy. We measured primary production by understory algae and phytoplankton approximately monthly from May 2007 through September 2008 at two locations on the reef: at the offshore edge of the forest in a patch with sparse Macrocystis and a lush assemblage of understory macroalgae (area ~ 1000 m2) and at 30 m inshore of the offshore edge, under a dense Macrocystis canopy with a sparse understory assemblage. Macrocystis was beginning to re-invade the location at the offshore edge, and we forestalled this by removing Macrocystis in March 2007, two months before the study began and by maintaining this area clear of Macrocystis throughout the study period.

Both locations, referred to as the Macrocystis removal (MR) and Macrocystis control (MC) sites, were at similar depth and in areas of relatively flat, low-relief rock substrate. The large influence of Macrocystis on light and flow in the Mohawk kelp forest coupled with the close proximity and similar biological and physical features of the Macrocystis removal and Macrocystis control sites greatly reduced the chance that factors other than the presence of giant kelp would cause differences in NPP between the two sites.

Understory algal biomass, production, and community structure

Primary production by the understory algal assemblage was measured each month along permanent 30 m transects at both the Macrocystis control and Macrocystis removal sites. Primary production was estimated from oxygen evolution measured in tunnel-shaped closed chambers (25 cm wide x 40 cm long x 40 cm tall), that consisted of two U-shaped end walls made of clear rigid acrylic, with continuous side walls and ceiling made of flexible Teflon sheeting (Tefzel, DuPont) and an open bottom framed by fiberglass-reinforced plastic that was sealed to the sea floor by a nylon gasket and a weighted flexible plastic skirt (Miller et al. 2009). Observations using rhodamine dye indicated that this made a highly effective seal (Miller et al. 2009). The flexibile side walls and ceiling permitted wave energy to be transmitted through the walls of the chambers, allowing macrophytes inside to oscillate naturally with wave-generated flow (sensu Gust 1977; Yates and Halley 2003). Chamber volume was ~45 L, and was measured for each chamber for production calculations. Water circulation was provided with a battery-powered submersible pump to ensure mixing of oxygen and prevent mass-transfer limitation of algal photosynthesis, (Rule 500 baitwell pump, 1890 L h-1). Self-contained optical probes (D-Opto, ENVCO) within each chamber logged dissolved oxygen concentration and temperature inside the chambers at a frequency of once per minute.

Oxygen measurements were taken simultaneously in four chambers per sampling date. Two chambers were placed randomly by divers along the control and removal transects on the morning of each sampling date and incubated for the majority of the day to capture diurnal variability in production rates (10 hours March-November, 8 hours December-February). The placement of chambers along the transects differed on each sampling date such that no plot was sampled more than once during the study. Conditions within the chambers were alternated between ambient light and dark on an hourly basis with darkness created by draping blackout cloth over the chamber. Chambers were flushed with outside water after each pair of light and dark incubations (every 2 h) by opening two stoppered ports and using the circulation pump to exchange water for 10 minutes. Oxygen saturation levels inside the chambers never exceeded those of ambient seawater by more than 10%.

After all incubations for the day were complete, the biomass of macroalgae enclosed by the chambers was collected by gently scraping it from the bottom into a fine mesh bag. Macroalgae were separated from animals and other material in the laboratory and sorted by taxa. The algae were cleaned of animal epiphytes, blotted dry, weighed and then dried at 60oC and re-weighed to obtain dry weight. Subsamples of dried tissue of each algal taxon were ground and analyzed for carbon content, and these values were used to convert dry weight to carbon weight. To evaluate the potential for differences in nutrient availability between the Macrocystis removal and Macrocystis control sites, tissue samples from species of understory algae that were present within at least one chamber at both sites on the same date were also analyzed for carbon and nitrogen content using a Carlo-Erba (Flash EA 1112 series) automated organic elemental analyzer.

Production and respiration rates were calculated by plotting oxygen concentration over incubation time, fitting a linear regression line to the data, and using the regression equation to calculate hourly rates of oxygen change. Net community production (NCP) was estimated as oxygen production in the light incubation. We report estimates of NCP per 12 hour period, since we did not measure nighttime community respiration, which is likely to be lower and non-linearly related to daytime respiration (Barron and Duarte 2009). Oxygen consumption in the dark incubation was used as an estimate of community respiration (CR). Gross primary production (GPP) was calculated as the sum of oxygen produced in the light and that consumed by respiration in the dark, and GPP was converted to NPP using mass-specific respiration rates obtained in laboratory dark incubations of 11 of the common understory species representing >97% of the understory biomass (R.J. Miller unpublished data). Briefly, algal specimens were collected and kept in running seawater, and within 1 day were incubated for 30 minutes in the dark to obtain respiration rates, after which the algae were dried at 60oC and weighed. For rare species that were not measured, average respiration rates of related species were used to estimate NPP. Middelboe et al. (2006) found that separate measurements of thallus respiration, weighted by biomass, accurately predicted community respiration in mixed-species heterotrophy-free algal assemblages. While we do not know the degree to which respiration in the light is different from respiration in the dark, the carbon-concentrating capacity of aquatic plants should minimize this difference (Falkowski and Raven, 2007). Direct measures of the effect of oxygen concentration on DIC uptake and compensation point in marine macroalgae have supported this view (Cook and Colman 1987).

NPP, NCP, and CR were integrated across the day for each plot, using daylengths calculated with sunrise and sunset times for each sampling date, and are expressed in mg C m-2 d-1, which was obtained using photosynthetic and respiratory quotients of 1, following Rosenberg et al. (1995), who found that PQ did not consistently relate to nutrient source or taxonomy and recommend using a value of 1 for the sake of inconvertibility when DIC uptake is not directly measured. Near-bottom rates of phytoplankton NPP were low, averaging less than 1% of benthic production, and were not corrected for, since measurements of phytoplankton and benthic algae were not simultaneous.

Phytoplankton biomass and production

Phytoplankton production was measured at the Macrocystis removal and Macrocystis control sites using in situ 13C-bicarbonate tracer incubations according to the methods of Shipe and Brzezinski (2003). Briefly, a pair of 500 mL polycarbonate light and dark bottles were filled with water collected at 5 depths (1, 2, 3, 4, 6 m) at each site (Macrocystis control and Macrocystis removal) using an 8-L Go-Flo bottle (General Oceanics). Following addition of 0.5 ml of 0.167 mol L-1 H13CO3- (99.9 atom%), experimental bottles were incubated for ~24 h at each site on a moored line at the collection depths, placed in a dark cooler upon collection, and filtered through precombusted (450oC for 2 h) glass fiber filters. 13C atom percentage of the particulate matter was measured using a Thermo Finnigan Delta-Plus Advantage isotope mass spectrometer coupled with a Costech EAS elemental analyzer. Carbon fixation in the incubation bottles was calculated as:

where A%sam is atom percent 13C measured on the filtered sample after incubation, A%nat is the average natural abundance of 13C in suspended particulate organic carbon (POC, 1.112%, Fernandez et al. 2005), and A%enr is the atom percent 13C of the labeled substrate. POC0 is the pre-incubation concentration of POC (μmol C L-1). Production was corrected for dark uptake, including any that occurred between collection and filtration, and integrated through the water column (to 6 m depth). 24-h carbon tracer uptake best represents NPP, although high heterotrophic activity during the night may lead to an underestimation of production (Marra 2009). POC and chlorophyll a (Chl a) concentrations (surrogates for phytoplankton biomass) were measured for each sampling depth. POC concentrations were measured in 630 mL water samples filtered through precombusted glass fiber filters and analyzed with a Leeman Labs (Model 440) carbon-hydrogen-nitrogen analyzer. Chl a was measured in 200 mL water samples filtered through 0.45 m, 47 mm cellulose ester Millipore filters following Parsons et al. (1984).