Richard C. Zimmerman

Impacts of CO2 Enrichment on Productivity and Light Requirements of Eelgrass.

Seagrasses, although well adapted for submerged existence, are CO2-limited and photosynthetically inefficient in seawater. This leads to high light requirements for growth and survival and makes seagrasses vulnerable to light limitation. We explored the long-term impact of increased CO2 availability on light requirements, productivity, and C allocation in eelgrass (Zostera marina L.). Enrichment of seawater CO2 increased photosynthesis 3-fold, but had no long-term impact on respiration. By tripling the rate of light-saturated photosynthesis, CO2 enrichment reduced the daily period of irradiance-saturated photosynthesis (Hsat) that is required for the maintenance of positive whole-plant C balance from 7 to 2.7 h, allowing plants maintained under 4 h of Hsat to perform like plants growing in unenriched seawater with 12 h of Hsat. Eelgrass grown under 4 h of Hsat without added CO2 consumed internal C reserves as photosynthesis rates and chlorophyll levels dropped. Growth ceased after 30 d. Leaf photosynthesis, respiration, chlorophyll, and sucrose-phosphate synthase activity of CO2-enriched plants showed no acclimation to prolonged enrichment. Thus, the CO2-stimulated improvement in photosynthesis reduced light requirements in the long term, suggesting that globally increasing CO2 may enhance seagrass survival in eutrophic coastal waters, where populations have been devastated by algal proliferation and reduced water-column light transparency.

Episodic Nutrient Supply to a Kelp Forest Ecosystem in Southern California

Temporal patterns of nutrient input into a Southern California kelp forest were measured using traditional hydrocast sampling coupled with high frequency temperature profiling. Patterns of nutrient input were related to growth rates of Macrocystis pyrifera located in an adjacent kelp forest. There were 2 distinct components to the pattern of nutrient availability. The long term, or seasonal, component was consistent with large-scale storm-induced mixing and horizontal advection during winter months. In addition, vertical motions of the thermocline, bringing nutrients into the kelp forest, occurred throughout the year with a frequency of about 2 per day and were strongest during the summer months. Weekly hydrocast sampling methods were inadequate for measuring these episodic events, and high frequency sampling was required to resolve the pattern of nutrient input accurately. Although measurable, nutrient input from vertical thermocline motion was inadequate to sustain maximum growth of Macrocystis pyrifera at 10m depth during the summer months. Thus, the major component of nutrient input came during the winter. These results indicate that nitrate limitation of M. pyrifera is a likely cause of reduced summer growth. Further, high frequency sampling is necessary to predict nutrient availability in nearshore ecosystems dominated by benthic macrophytes where the pattern of nutrient input is dominated by episodic events of short duration.

Eelgrass (Zostera marina L.) transplants in San Francisco Bay: Role of light availability on metabolism, growth and survival

Survival, metabolism and growth of Zostera marina L. transplants were examined along depth gradients in Keil Cove and Paradise Cove in the extremely turbid San Francisco Bay estuary. Water transparency was unusually high throughout 1989–1990 for San Francisco Bay. Transplant survival was strongly depth-dependent at Paradise Cove but not at Keil Cove. All transplants were lost below − 1.0 m depth within 1 year at Paradise Cove, but survived to depths of − 1.5 m at Keil Cove. Half the transplants growing in shallow water survived the first year at both sites. Shoot photosynthesis, respiration, growth, and sugar content did not differ between sites. Daily periods of irradiance-saturated photosynthesis (Hsat) were over 6 h all year. Seasonal photosynthetic acclimation to light availability maintained long Hsat periods and high ratios of daily whole-plant production to respiration through the winter, indicating a potential for net carbon gain throughout the year. Winter growth was 50% of the summer rate. Despite high initial losses, surviving transplants have persisted at both sites through 1994. Although eelgrass transplants can succeed in San Francisco Bay given sufficient light availability, the role of carbon reserves and transplant timing may influence transplant survival.

Thermal acclimation and whole-plant carbon balance in Zostera marina L. (eelgrass)

Thermal acclimation in eelgrass Zostera marina L. was investigated in laboratory experiments after growing plants at 10 and 20°C for 21 days under a 12:12 L:D regime. Metabolic rates showed significant shifts in short-term response to temperature in leaves and roots. Growth rates, tissue carbohydrate concentrations and metabolic rates measured at the two growth temperatures were statistically identical, indicating that thermal acclimation was essentially complete at these temperatures. When measured at pO2 values high enough to achieve capacity rates of respiration, thermal responses of respiration (Q10) were lower than previously reported while the thermal response of photosynthesis (measured at pO2 below air saturation) was similar to previous reports. Daily C budgets constructed from metabolic rate data indicated that Hsat periods required for photosynthesis to balance C demand can vary from 3 to > 12 h, depending on the ratios of net photosynthesis: respiration (Pnet: R) and shoot: root. Since Z. marina. shows evidence of thermal acclimation, seasonal changes in ambient temperature may not significantly affect Hsat requirements and whole-plant C balance. Rapid mortality at high temperatures during summer may result instead from thermal disruption of metabolism while internal C reserves may be important in meeting C demand during winter periods of low light availability, particularly among high-latitude populations.

Modeling underwater light climate in relation to sedimentation, resuspension, water quality and autotrophic growth

The underwater light climate ultimately determines the depth distribution, abundance and primary production of autotrophs suspended within and rooted beneath the water column. This paper addresses the underwater light climate, with reference to effects of suspended solids and growth responses of autotrophs with emphasis on phytoplankton.Effects of the most important factors contributing to the absorption and scattering of light in surface waters were described. A comparison between spectral and scalar approaches to underwater light climate modeling was made and examples of linear approximations to light attenuation equations were presented. It was demonstrated that spectral and scalar photosynthesis models may converge to similar values in spectral-flat, high photon flux environments, but that scalar PAR models may overestimate biomass-specific production by 70%. Such differences can lead to serious overestimates of habitat suitability for the growth and survival of submersed macrophytes, particularly in relatively turbid, coastal waters.Relationships between physical and optical properties of suspended sediments were described theoretically, and illustrated with modeling examples and measurements. It was found that the slowly settling particulate fraction contributed substantially to the suspended solids concentration, and greatly to light attenuation within the water column. It was concluded that distinguishing particles by fall velocity and concomitant light attenuation properties in the modeling of underwater light conditions allowed the establishment of useful, although not simply linear, relationships.In eutrophic, shallow lakes, the largest contribution to light attenuation often originates from phytoplankton on a seasonal basis (months–years), but from suspended solids behavior on a shorter time scale (days–weeks), particularly when water bodies are wind-exposed. Temporal and spatial variabilities in wave height, suspended solids concentrations, and light attenuation within the water column, and their importance for autotrophic growth were described, and illustrated with a case study pertaining to Markermeer, The Netherlands. The influence of underwater light conditions on phytoplankton succession was briefly discussed and illustrated with a case study pertaining to Lake Veluwe, The Netherlands. It was concluded that modeling the underwater light climate in a water body on a few sites only can indicate how important various components are for the attenuation of light, but based on the current state of the art, it can not be expected that this will provide accurate predictions of the underwater light climate, and of phytoplankton and submersed macrophyte growth.

The antifouling activity of natural and synthetic phenolic acid sulphate esters

p-(sulphooxy) Cinnamic acid was isolated as a natural product for the first time from the seagrass Zostera marina (eelgrass) and was found to prevent attachment of marine bacteria and barnacles to artificial surfaces at nontoxic concentrations. Analogous synthetic sulphate esters had similar antifouling properties, while the non-sulphated phenolic acid precursors were ineffective. The antifouling properties of phenolic acid sulphates are consistent with an emerging pattern of biological activity exhibited by other sulphate esters isolated from a variety of marine organisms, and their low toxicity offers promise for the development of environmentally benign antifouling agents to protect structures in aquatic environments.

Inorganic carbon sources for seagrass photosynthesis: an experimental evaluation of bicarbonate use in species inhabiting temperate waters

Abstract Photosynthetic inorganic carbon utilization by the seagrasses Posidonia oceanica (L.) Delile, Cymodocea nodosa (Ucria) Aschers., Zostera marina (L.) and Phyllospadix torreyi S. Watson was studied by manipulating the concentrations of aqueous dissolved CO2 and HCO3− in seawater. At constant dissolved inorganic concentration (ca. 2.2 mM), photosynthetic rates were higher at low pH (i.e. 6–7; [CO2]=1.2–0.21 mM) than at “normal” pH (i.e. 8.2; [CO2]=0.016 mM) in all four species. Photosynthetic rates of all four species exhibited saturation kinetics in response to [HCO3−] at constant, low [CO2] (0.016 mM), with saturating [HCO3−] between 1 and 1.5 mM; this demonstrates, as it is known for other seagrass species, a clear capacity to use HCO3− as inorganic carbon source for photosynthesis. Photosynthetic rates at saturating [HCO3−] were 1.6 to 6 times lower than rates measured at high (∼1 mM) [CO2]. Thus, photosynthesis of these species was limited by dissolved inorganic carbon (DIC) availability in normal seawater. No significant differences were found among species in their affinity for HCO3− as determined by the half-saturation constant Ks. [CO2(aq)] enhancement of photosynthesis was lower in the Mediterranean species (P. oceanica and C. nodosa) than the Pacific ones (Z. marina and P. torreyi); in addition, P. oceanica showed the highest HCO3−-driven photosynthesis at saturating HCO3− concentration. These species-specific differences in inorganic carbon uptake kinetics should be considered in any inter-specific analysis of plant features depending on carbon balance, e.g. the deep distribution limit or the sensitivity to future changes in [CO2(aq)]. However, CO2 limitation of seagrass photosynthesis appears to be a common physiological feature of seagrasses that may have significant ecological and evolutionary implications.

Assessment of environmental suitability for growth of Zostera marina L. (eelgrass) in San Francisco Bay

Abstract The relationship between turbidity and light availability, and its subsequent effect on the depth distribution of Zostera marina L. (eelgrass) in San Francisco Bay was explored. The average daily period of irradiance-saturated photosynthesis (Hsat) required for the maintenance of whole plant carbon balance and growth, based on measured rates of photosynthesis and respiration as well as data available in the literature, were estimated to be between 3 and 5 h. Estimates of average Hsat availability in the field were determined from laboratory measurements of the phytosynthesis vs. irradiance (P vs. I) response and from field observations of light attenuation measured at five sites in San Francisco Bay. Although plants were found to be low-light adapted with regard to their P vs. I response, they were limited to depths shallower than −2 m mean lower low water (MLLW) at all sites. The −2 m depth limit corresponded fairly well to predicted Hsat requirements at two sites where turbidity was relatively low and constant, but depth limits of eelgrass were less than 1.5 m at three sites subjected to extremely turbid and variable light environments, much shallower than predicted from calculations of mean daily carbon requirements and Hsat availability. Thus, in addition to the mean light environment, periods of extreme light attenuation that last from days to weeks may be important in controlling eelgrass growth and productivity in highly turbid and dynamic estuaries such as San Francisco Bay.

Top-down impact through a bottom-up mechanism : the effect of limpet grazing on growth, productivity and carbon allocation of Zostera marina L. (eelgrass)

The unusual appearance of a commensal eelgrass limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot−1) has coincided with the catastrophic decline of a subtidal Zostera marina L. meadow in Monterey Bay, California. Some commensal limpets graze the chloroplast-rich epidermis of eelgrass leaves, but were not known to affect seagrass growth or productivity. We evaluated the effect on eelgrass productivity of grazing by limpets maintained at natural densities (8±2 shoot−1) in a natural light mesocosm for 45 days. Growth rates, carbon reserves, root proliferation and net photosynthesis of grazed plants were 50–80% below those of ungrazed plants, but biomass-specific respiration was unaffected. The daily period of irradiance-saturated photosynthesis (Hsat) needed to maintain positive carbon balance in grazed plants approached 13.5 h, compared with 5–6 h for ungrazed plants. The amount of carbon allocated to roots of ungrazed plants was 800% higher than for grazed plants. By grazing the chlorophyll-rich epidermis, T. depicta induced carbon limitation in eelgrass growing in an other-wise light-replete environment. Continued northward movement of T. depicta, may have significant impacts on eelgrass production and population dynamics in the northeast Pacific, even thought this limpet consumes very little plant biomass. This interaction is a dramatic example of top-down control (grazing/predation) of eelgrass productivity and survival operating via a bottom-up mechanism (photosynthesis limitation).

Light harvesting and the package effect in the seagrasses Thalassia testudinum Banks ex König and Zostera marina L.: optical constraints on photoacclimation

Although seagrasses possess numerous adaptations for life underwater, they lack the specialized accessory pigments for the efficient harvesting of green light that dominates many aquatic environments. Without these specialized pigments, photoacclimation by seagrasses is likely to result in a severe package effect, i.e. the loss of linearity between light harvesting efficiency and pigment loading. Here, we investigated the optical constraints imposed by the package effect on photoacclimation in seagrass leaves. Pigment concentrations and optical properties (absorptances, absorption coefficients and optical cross-sections) of turtlegrass ( Thalassia testudium) leaves from Lee Stocking Island (LSI; Bahamas) and eelgrass (Zostera marina) leaves from Monterey Bay, CL, USA were measured at different times of the year. Chlorophyll concentrations and optical cross-sections differed by a factor of five across sampling dates and populations. Increases in leaf-specific absorption among seagrass leaves were greatest in the green (500–600 nm), while the package effect, as measured by a decrease in leaf optical cross-section, was most severe in the blue (400–500 nm) and red (600–700 nm). Consequently, the five-fold range in pigment concentration resulted in similar photosynthetic light harvest efficiencies ( φL ≈ 50% of incident PAR) for intact seagrass leaves in their native light environments. Although the package effect has significant impacts on the optical properties of seagrass leaves, chlorophyll use efficiency does not appear to play a strong role in the ecology or evolution of seagrasses.