Seagrasses, although well adapted for submerged existence, are C0,-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 CO, availability on light requirements, productivity, and C allocation in eelgrass (Zostera marina 1.1. Enrichment of seawater CO, increased photosynthesis 3-fold, but had no longterm impact on respiration. By tripling the rate of light-saturated photosynthesis, CO, 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,

This study evaluated the abihty ot Zostera marma L (eelgrass) to balance the daily photosynthetic deflc~t by mobilization of carbon reserves stored In below-ground tlssues durlng a period of extreme w n t e r llght lln~itation A quantitative understand~ng of the mobllizat~on process and its hmltatlons is essential to the development of robust models predict~ng mnimuni llght levels r e q u~r e d to nialntaln healthy seagrass populatlons Plants were grown m runnlng seawater tanks under 2 llght reglrnes One treatment was provlded wlth 2 h irradiance-saturated photosynthes~s (H,,,) to produce severe llght limitation, whlle control plants were grown under 7 h H,,, simulating the typical w~n t e rtune condition In Monterey Bay, California, USA Although plants maintained under 2 h H,,, were more severely carbon h m t e d than plants grown under 3 h H,,, whole-plant carbon balance calculated from metabohc needs and growth rates was negative for both Q,, treatments The eelgrass studied here responded to negative carbon balances by suppressing the product~on of new roots, depleting sucrose reserves and effecting a gradual decrease In growth rate a n d a n Increase In the activlty of sucrose synthase (SS, E C 2 4 1 13) in s~n k tlssues in the termnal stages of carbon stress The 3 h H,,, plants survlved the 45 d course of the expenment while the plants grown under 2 h H,,, died within 30 d even though one-thlrd of the11 carbon reserves remalned lmmobdized In the rhizome Thus extreme l~g h t llmitatlon can prevent full mob~llzation of carbon reserves stored in below-ground tissues probably through the effects of anoxla on translocation Metabolic rates, particularly photosynthesls and resplration of the shoot, were unaffected by prolonged carbon hrmtation in both treatments The patterns observed here can provide useful indices for assess~ng the state and fate of seagrass ecosystems in advance of catastrophic declines KEY WORDS: Seagrass Carbon balance . Resource a l l~c d l i~~n .Photosynthesis . Light

Diel variations in rates of C export, sucrose-phosphate synthase (SPS) and sucrose synthase (SS) activity, and C reserves were investigated in Zosfera marina L. (eelgrass) to elucidate the environmental regulation of sucrose formation and partitioning in this ecologically important species. Rates of C flux and SPS activity increased with leaf age, consistent with the ontogenic transition from sink to source status. Rates of C export and photosynthesis were low but quantitatively consistent with those of many terrestrial plant species. The V,,, activity of SPS approached that of maize, but substrate-limited rates were 20 to 25% of V,,,, indicating a large pool of inactive SPS. SPS was unresponsive to the day/night transition or to a 3-fold increase in photosynthesis generated by high [CO,] and showed little sensitivity to inorganic phosphate. Consequently, regulation of eelgrass SPS appeared similar to starch-rather than to sugar-accumulating species even though eelgrass accumulates sucrose. Leaf [sucrose] was constant and high throughout the diel cycle, which may contribute to the down-regulation of SPS. Root sucrose synthase activity was high but showed no response to nocturnal anoxia. Root [sucrose] also showed no diel cycle. l h e temporal stability of [sucrose] confers an ability for eelgrass to buffer the effects of prolonged light limitation that may be key to its survival and ecological success in environments subject to periods of extreme light limitation and chaotic daily variation in light availability.

The unusual appearance of a commensal eelgrass limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot) 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) 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 (H ) 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).

Temporal changes in abundance, size, productivity, resource allocation and light requirements of a subtidal eelgrass (Zostera marina L.) population were followed for 2 yr after the September 1993 appearance of a previously rare oval form of the commensal limpet Tectura depicta (Berry) in Monterey Bay, California, USA. By exclusively targeting the epidermis, limpet grazing impaired photosynthetic performance but left respiratory demand, meristematic growth and more than 90% of the leaf biomass intact. The resulting low P :R ratios of grazed plants raised the light requirements for the maintenance of positive carbon balance almost 2-fold relative to healthy ungrazed plants and prevented the summertime accumulation of internal carbon reserves. Shoot density in this once-continuously vegetated 30 ha meadow declined from more than 50 shoots m ). More than 50% of the continuously vegetated meadow was converted to bare sand despite ambient light availability and water temperatures that were favorable for growth of healthy, ungrazed plants. Plant size declined by 50% and internal sugar reserves declined more than 4-fold within 6 mo after the appearance of T. depicta. Plant losses were most extensive during winter, when internal carbon reserves were minimal. The dramatic decline in eelgrass vigor and abundance reported here, despite a physical environment that was favorable for healthy eelgrass survival, illustrates the amplification of top-down control by this relatively inconspicuous limpet through a feeding mechanism that specifically impairs photosynthesis, a bottom-up process.