Causes of sulfur isotope variability in the seagrass, Zostera capricorni

Sulfur has been proposed as a useful element to employ in addition to carbon and nitrogen in stable isotope studies of marine food webs, but variability in δ³⁴S of primary producers may prevent food web resolution. δ³⁴S values in green leaves of the seagrass, Zostera capricorni, showed considerable variability (12.7-17.6‰) in a survey in Moreton Bay, Australia. We demonstrated that δ³⁴S values were correlated with sediment organic matter (OM) content and height of seagrass on the tidal gradient, but these relationships were opposite to those expected from work elsewhere. In our survey, δ³⁴S values were relatively depleted at sites higher on the shore and with lower OM content. We did find the expected relationship of depleted δ³⁴S values where sediment porewater sulfide concentrations were higher. Any influence of OM content on δ³⁴S values would have been confounded in the survey by the relationship between height on shore and OM content itself. We separated the effects of height and OM content by creating the following treatments at one height on the shore: (1) OM added, (2) procedural control, and (3) untouched control. δ³⁴S values of seagrass in OM added plots were significantly depleted (5.6‰) relative to procedural (10.1‰) and untouched (11.0‰) controls 8 weeks after the manipulation. This demonstrated that OM content on its own does have the expected effect on δ³⁴S values of seagrass, so in the initial survey another factor, probably related to height on shore, must have overridden the influence of OM content. Seagrass roots are able to exude excess oxygen produced during photosynthesis, reoxidising sulfides in surrounding porewater. We demonstrated that the above and below-ground biomass of seagrass was higher low on the shore, and contend that higher seagrass productivity low on the shore results in greater reoxidation of sulfides and leads to more enriched δ³⁴S values of seagrass.     

Aerial and underwater carbon metabolism of a Zostera noltii seagrass bed in the Banc d’Arguin, Mauritania

Community respiration and primary production were measured in a dense intertidal Zostera noltii bed on the Banc d’Arguin, Mauritania (West Africa) under aerial and submerged conditions. Metabolism was studied in situ in dark and transparent benthic chambers. CO₂ fluxes in the air were measured over a series of short-term incubations (3 min) using an infrared gas analyzer. Dissolved inorganic carbon fluxes were calculated from concentration changes during one-hour underwater incubations. Air and underwater irradiance levels were measured every minute throughout the experiments. Carbon respiration was lower in the air (2.2 mmol m⁻² h⁻¹) than underwater (5.0 mmol m⁻² h⁻¹); similarly, a production-irradiance model fitted to the data indicated that gross maximal photosynthetic rate was markedly lower during emergence (6.0 mmol C m⁻² h⁻¹) than under water (42.7 mmol C m⁻² h⁻¹). The δ¹³C values observed in shoots indicated a decrease in atmospheric CO₂ contribution, compared to dissolved inorganic carbon, in Z. noltii metabolism along a depth gradient within a single location. As the seagrass bed remains under a thin layer of water at low tide at the studied site, the large difference in primary production can be mainly attributed to photosynthesis inhibition by high pH and oxygen concentration, as well as to the negative feedback of self-shading by seagrass leaves during emersion. The observed differences in respiration can be explained by the oxygen deficit at night during low tide near the sediment surface, a deficit that is consistent with the abundance of anoxia-tolerant species.     

Timing and success of reproductive stages in the seagrass Zostera noltii

The timing and success of sexual reproduction of the seagrass Zostera noltii was investigated at the Ria Formosa lagoon, Portugal. Thirty plants were tagged and monitored individually through time to determine in situ the development time of each maturation stage, from the emergence of the flowers to the production of seeds. The overall process of flowering and fruiting lasted 47 ± 4 days, during which formation and maturation of the fruits was the most time-consuming stage (27 ± 2 days). Spathe success, i.e. the percentage of spathes that produced seeds, was 22 ± 4% while spathe mortality was 34 ± 6%. A considerable percentage of spathes (37 ± 7%) was lost through leaf detachment, but some of these may still add to the reproductive success of the species by contributing to the species recruitment within the source meadow or elsewhere. Meadow seed production (MSP) of Z. noltii was estimated to be 312 ± 66 seeds m⁻², whereas the potential seed production of the studied meadow was 2623 seeds m⁻². Under laboratory conditions, 70% of Z. noltii seeds germinated within 26 days, but only 10% reached the seedling stage due to the high mortality of germlings. Fertility, defined as the probability of a seed to originate a new plant, was estimated to be 14 × 10⁻⁴, which is higher than what is expected for most seagrasses.     

Variations in sensitivity to copper and zinc among three isolated populations of the seagrass, Zostera capricorni

Metal accumulation in seagrass is well documented, but toxic impacts and mechanisms of tolerance in seagrass are not well understood. We looked at the impacts of 10 h exposure to copper and zinc for three isolated populations of Zostera capricorni in the Sydney (Australia) region. Photosynthetic efficiency (measured as the effective quantum yield, ΔF/Fm′) and chlorophyll pigment concentrations showed different sensitivities to metal impacts at the three geographically isolated sites. Seagrasses from the least developed estuary were the most sensitive to metals and the two more developed estuaries had more tolerant populations. Determination of metal concentrations in the leaves showed that there was no difference in metal exclusion as the sensitive seagrass accumulated no more metal than the tolerant seagrass. Equally, background levels of copper and zinc in the sediments and seagrass tissue could not explain the differences in tolerance. We discuss some other possible mechanisms of tolerance. The outcomes suggest that assessing metal content in seagrass tissue may not demonstrate degree of photosynthetic impact.     

Operation of the xanthophyll cycle in the seagrass Zostera marina in response to variable irradiance

Changes in the photobiology and photosynthetic pigments of the seagrass Zostera marina from Chesapeake Bay (USA) were examined under a range of natural and manipulated irradiance regimes. Photosynthetic activity was assessed using chlorophyll-a fluorescence, and photosynthetic pigments were measured by HPLC. Large changes in the violaxanthin, zeaxanthin, and antheraxanthin content were concomitant with the modulation of non-photochemical quenching (NPQ). Photokinetics (F_v/F_m, rapid light curves (RLC), and non-photochemical quenching) varied as a result of oscillating irradiance and were highly correlated to xanthophyll pigment content. Zeaxanthin and antheraxanthin concentrations increased under elevated light conditions, while violaxanthin increased in darkened conditions. Unusually high concentrations of antheraxanthin were found in Z. marina under a wide range of light conditions, and this was associated with the partial conversion of violaxanthin to zeaxanthin. These results support the idea that xanthophyll intermediate pigments induce a photoprotective response during exposure to high irradiances in this seagrass.     

Can chlorophyll fluorescence be used to estimate photosynthetic production in the seagrass Zostera noltii?

Seagrass photosynthesis is usually measured in laboratory experiments, following oxygen evolution in closed chambers. Pulse amplitude modulated (PAM) fluorometry constitutes an alternative and non-intrusive method of measuring photosynthesis in the field. Validation of electron transport rate (ETR) measurements as reliable estimators of actual photosynthetic production requires that a significant linear relationship between oxygen production and ETR is demonstrated, and also that a 0.25 molar ratio between these two measures is verified. In this work, both parameters were measured simultaneously in laboratory experiments, over a range of light intensities, under well-defined and controlled conditions. A linear relationship was observed between the average rates of oxygen production and the electron transport rates for Zostera noltii (Hornemann) obtained at several irradiances. The molar ratio found between oxygen production and ETR was 0.15±0.02, lower than the theoretically expected value of 0.25. The use of PAM fluorescence as a valid proxy for photosynthetic production was validated for the range of 35-490 μmol photons m⁻² s⁻¹ (PAR), under the assumption that the electron sinks responsible for the molar ratio deviation remain constant in similar experimental conditions.     

The ecology of eelgrass, Zostera marina (L.), fish communities. I. Structural analysis

Fish populations utilizing eelgrass, Zostera marina (L.), beds in two different estuarine areas near Beaufort, North Carolina were analysed and compared to determine aspects of their community structure. The fish community of the eelgrass beds was characterized by low diversity and high standing crops of biomass and energy, both of which showed seasonal variation. Wide temperature fluctuations related to the overall shallowness of the beds probably regulated the diversity of fishes utilizing the beds. This community was dominated by pinfish, Lagodon rhombiodes (L.), which comprised 45% and 67% of the fish biomass in the Phillips Island and Bogue Sound beds, respectively.Changes in total body caloric content were probably related to developmental stages and changes in diet. Adult fish often had significantly higher weight-specific caloric contents than juvenile fish. Monthly or seasonal variations in caloric content of the organic matter of pinfish had little influence on the caloric content within the various sizes of pinfish.There was a significant correlation between fish biomass, temperature, and Zostera biomass. Fish biomass was highest when temperature and grass biomass were at a maximum. In general, water depth over the beds had little effect on the standing crop of fish within the bed, but cooler waters which occurred at night, darkness, or both, had a large effect.     

Field studies on seed germination and seedling development in Zostera marina L.

Seeds of Zostera marina L. were collected in July 1978 and July 1979 and planted within two weeks in sediment-filled containers in shallow water at Northwest Creek, New York. The containers were retrieved at periodic intervals over the next 12 months and the seeds and seedlings examined. Between 76 and 93% of the recovered seeds germinated; primarily during October–December, 3–4 months after release. Seedling growth and development occurred during the autumn and spring, but not in winter. A progressive loss of seedlings was observed during the winter and spring, however, and none survived past May.Three stages of seedling development were distinguished. Stage 1 represented germination including emergence and straightening of the cotyledon. Stage 2 seedlings exhibited continued growth of the cotyledon and variable, but often extensive, elongation (2–50 mm) of the axial hypocotyl. The plumule remained undeveloped (length ? 3 mm), and there were no hairs on the basal hypocotyl. Stage 3 seedlings showed the same range in axial hypocotyl length, but they were distinguished by the onset of plumule growth and hypocotyl hair formation.     

The ecology of eelgrass, Zostera marina (L.), fish communities. II. Functional analysis

Consumption, production, and respiration of fish communities utilizing two eelgrass beds in a shallow estuarine system near Beaufort, North Carolina have been estimated for 1971–1972: annual production was 21.7 kcal/m² in each bed with pinfish accounting for 45 and 68% of the production in the Phillips Island and Bogue Sound beds, respectively. Annual community respiration was 57.9 and 69.7 kcal/m² in the two beds with pinfish accounting for 62.6 and 26.7% of the total in the Bogue Sound and Phillips Island beds, respectively. Estimation of the annual food energy consumed by the eelgrass fish community using the Winberg and daily ration methods gave values within 6% of each other.Energy turnover was high (2.8), and the efficiency of energy dissipation low for the two eelgrass fish communities, suggesting that the resident fish populations were adapted to the temperature extremes within the bed. High ecological efficiencies of 0.24 and 0.23 and the high overall efficiency of the eelgrass system (production/solar radiation) of 0.0051 and 0.0086% indicate that the eelgrass beds are efficient systems for converting consumed energy and solar radiation into fish.     

Ammonium uptake kinetics in root and leaf cells of Zostera marina L.

Ammonium uptake rates and the mechanism for ammonium transport into the cells have been analysed in Zostera marina L. In the cells of this species, a proton pump is present in the plasmalemma, which maintains the membrane potential. However, this seagrass shows a high-affinity transport mechanism both for nitrate and phosphate which is dependent on sodium and is unique among angiosperms. We have then analysed if the transport of another N form, ammonium, is also dependent of sodium. First, we have studied ammonium transport at the cellular level by measurements of membrane potentials, both in epidermal root cells and mesophyll cells. And second, we have monitored uptake rates in whole leaves and roots by depletion experiments. The results showed that ammonium is taken up by a high-affinity transport system both in root and leaf cells, although two different of kinetics could be discerned in mesophyll cells (with affinity constants of 2.2 ± 1.1 μM NH₄⁺, in the range 0.01-10 μM NH₄⁺, and 23.2 ± 7.1 μM NH₄⁺, at concentrations between 10 and 500 μM NH₄⁺). However, only one kinetic could be observed in epidermal root cells, which showed a K_m = 11.2 ± 1.0 μM NH₄⁺, considering the whole ammonium concentration range assayed (0.01-500 μM NH₄⁺). The higher affinity of leaf cells for ammonium was consistent with the higher uptake rates observed in leaves, with respect to roots, in depletion experiments at 10 μM NH₄⁺ initial concentration. However, when an initial concentration of 100 μM was assayed, the difference between uptake rates was reduced, but still being higher in leaves. Variations in proton or sodium-electrochemical gradient did not affect ammonium uptake, suggesting that the transport of this nutrient is not driven by these ions and that the ammonium transport mechanism could be different to the transport of nitrate and phosphate in this species.     

Effects of light availability on growth, architecture and nutrient content of the seagrass Zostera noltii Hornem

The growth vs. irradiance response of the seagrass Zostera noltii from Cadiz Bay Natural Park (southwestern Spain) was characterised. Plants were exposed along 14 days to different light treatments (1%, 7%, 42% and 100% surface irradiance, SI), using shade screens in an outdoor mesocosm. Growth at 100% SI (1.6 mg DW plant⁻¹ day⁻¹) was lower than that at 42% SI (2.4 mg DW plant⁻¹ day⁻¹), suggesting photoinhibition. The minimum light requirement estimated was 0.8 mol photons m⁻² day⁻¹ (2% SI). Light availability affected the pattern of plant development and the overall plant growth. The contribution of the apical shoots to the aboveground production was nearly constant (c.a. 1.13 cm plant⁻¹ day⁻¹) regardless of the light level (except at 1% SI). In contrast, recruitment and growth of lateral shoots arising from the main rhizome axes accounted for the observed differences in aboveground growth. Rhizome branching was only observed at 42% SI. The possibility of a light threshold for rhizome branching could explain the seasonality of shoot recruitment, as well as the observed decrease in shoot density along depth (or light) gradients in seagrass meadows. Carbon demands at low irradiances (1% and 7% SI) were partially met by mobilization of carbohydrate reserves (sucrose in belowground and starch in aboveground parts). Plant nitrogen content decreased with increasing light, especially in belowground parts, reaching critical levels for growth.     

Assessing within habitat variability in plant demography, faunal density, and secondary production in an eelgrass (Zostera marina L.) bed

This investigation addressed faunal relationships with habitat structure within a Zostera marina community targeting differences between seagrass bed edge and interior. Z. marina biomass was significantly higher from the interior portions of the bed compared to the edge, but shoot density did not vary. Additionally, leaf width and length were significantly greater in the interior of the bed, suggesting greater total leaf area. Densities of larger organisms (> 0.85 mm) were significantly greater in vegetated samples (Z. marina edge and interior) compared to unvegetated, but an analysis of similarities demonstrated significant faunal community differences among each of the identified habitats. Densities of small organisms (0.25-0.85 mm), however, were significantly greater at Z. marina edge compared to unvegetated samples and Z. marina interior. Additionally, secondary production (μg AFDW day^− 1) was estimated based on the size distribution of taxa and showed significantly greater production from samples gathered in Z. marina compared to unvegetated samples. The relative size distribution of taxa was assessed using regression analysis and results showed that the size distribution was similar for samples collected at edge and interior Z. marina, but these distributions differed significantly when compared to unvegetated samples. The results of this study suggest that although similarities exist between edge and interior portions of Z. marina beds, especially compared to unvegetated habitats, noteworthy differences in faunal density, species composition, size distribution, and secondary production exist between edge and interior Z. marina.     

Effects of light and biomass partitioning on growth, photosynthesis and carbohydrate content of the seagrass Zostera noltii Hornem

Plants of the seagrass Zostera noltii were cultured in the laboratory (mesocosms) for two weeks to assess the effect of above:below-ground (AG/BG) biomass ratios and light on growth, photosynthesis and chemical composition. Experimental plant units (EPUs) with different proportions between AG and BG biomass were obtained from plants of the same size (containing 6 shoots and 5 internodes) by excising 0-5 shoots. The EPUs maintained the proportions in AG/BG biomass ratios during the experiment. While growth rate was unaffected by biomass partitioning at high light, maximum growth at low light was recorded in plants with low AG/BG ratios. The production of shoots and rhizomes showed a compensatory morphological response depending on the initial AG/BG proportions regardless of the light level. While shoot production, estimated as shoot appearance rate, was high at low AG/BG ratios and minimal under high AG/BG values, rhizome production, estimated as internode appearance rate and internode elongation rate, was maximal under high AG/BG proportions and decreased towards lower AG/BG ratios. This rhizomatic response was observed for secondary rhizomes and not for primary ones. In contrast to morphological response, no significant differences were detected in maximum electron transport rates (ETRm) among the different shoots in the plant. However, mean values of ETRm in plants were affected by biomass partitioning and light. EPUs grown in low light increased the sucrose stored in shoots as the AG/BG biomass ratios decreased; however, EPUs grown at high light showed no effect of biomass partitioning on sucrose levels. In conclusion, shoots excision by experimental manipulation caused a compensatory morphological response in plants while photosynthetic performance remained almost unaffected.     

Phenology of eelgrass, Zostera marina L., along latitudinal gradients in North America

Eelgrass, Zostera marina L., was collected monthly from 1976 to 1979 and its phenology compared in the distributional range on both coastlines of North America. Each of three reproductive phenophases (initial appearance of macroscopically visible flower buds, initial anthesis, initial appearance of visible fruits) differed significantly among the sites in their dates of occurrence. Among the sites, flowering at the same latitude was later in the Atlantic than in the Pacific and flowering along a latitudinal gradient occurred increasingly later at more northern latitudes in the Atlantic than in the Pacific. Although the reproductive periodicity probably is controlled primarily by water temperature at the different sites, the variation in timing at the same latitude among Pacific and Atlantic sites indicates that Z. marina may include genotypes with different temperature requirements that are selectively adapted to different habitats along the latitudinal gradients.     

Macrophyte-epiphyte biomass and productivity in an eelgrass (Zostera marina L.) community

The biomass, productivity (¹⁴C), and photosynthetic response to light and temperature of eelgrass, Zostera marina L. and its epiphytes was measured in a shallow estuarine system near Beaufort, North Carolina, during 1974. The maximum of the biomass (above-ground) was measured in March; this was followed by a general decline throughout the rest of the year. The average biomass was 105.0 g dry wt m^?2; 80.3 g dry wt m^?2 was eelgrass and 24.7 g dry wt m^?2 was epiphytes. The productivity of eelgrass averaged 0.88 mg C g^?1 h^?1 which was similar to that of the epiphytes, 0.65 mg C g^?1 h^?1. Eelgrass and epiphyte productivity was low during the spring and early summer, gave a maximum during late summer and fall, and declined during the winter; this progression was probably due to environmental factors associated with tidal heights. On an areal basis, the average annual productivity was 0.9 g C m^?2 day^?1 for eelgrass and 0.2 g C m^?2 day^?1 for the epiphytes. Rates of photosynthesis of both eelgrass and epiphytes increased with increasing temperature to an asymptotic value at which the system was light saturated. Both eelgrass and epiphytes had a temperature optimum of < 29 °C. A negative response to higher temperatures was also reflected in biomass measurements which showed the destruction of eelgrass with increasing summer temperatures. The data suggest that the primary productivity cycles of macrophytes and epiphytes are closely interrelated.     

Taxonomy,morphology, and surface structure of Basidiobolus sp. isolate

The isolate under investigation was determined as Basidiobolus ranarum Eidam, 1886. Scanning electron micrographs elucidated the origin of a cap-like part of the conidiophore adhering to the conidium and showed new details of the surface of the fragment of conidiophore and proper papilla during their gradual separation. The attaching sac of nonattached anadhesive conidia was smooth and very turgid, but after attaching, there appeared depressions due to the loss of turgor.     

Seasonal distribution and changes in the nutritive quality of living,dead and detrital fractions of Zostera marina L.

Samples of eelgrass, Zostera marina L., were collected monthly from December 1974 through December 1975 in a shallow embayment near Beaufort, N.C., and separated into green leaves, dead leaves, and leaf detrital material. Each component was analysed for dry weight, organic matter, inorganic and organic carbon, nitrogen and amino compounds.The standing crop of green and dead blades reached a maximum in April through June, while detrital matter had peaks in December, April and July–September. Inorganic carbon in the three grass fractions showed seasonal variations similar to those observed for epiphytic biomass in previous years, and represented 14, 24, and 30% of the total carbon associated with the green and dead leaves and detrital fragments, respectively. Organic carbon represented a decreasing proportion of the dry weight of these three fractions on a dry weight basis while there was a significant increase in organic carbon on an ash-free dry weight basis in the detrital fragments relative to the head dead blades. During senescence there was a loss of nitrogen from the leaves and an increase in the nitrogen content of the organic matter of the detritus relative to the dead leaves. The latter suggests that there was microbial growth on the detritus and subsequent nitrogen immobilization from the surrounding medium. There were significant decreases of lysine, histidine, arginine, glycine, tyrosine, and glucosamine in the dead leaves relative to the detritus. The glucosamine, derived from N-acetyl-glucosamine, a product of murein which is a component of microbial cell walls, had a seasonal distribution similar to that of the epiphytic community and available inorganic nitrogen in the surrounding water. The relative proportions of N-acetyl-glucosamine, nitrogen and organic carbon were all higher in the fall and winter.     

Effects of aerobic versus anoxic conditions on glutamine synthetase activity in eelgrass (Zostera marina L.) roots: regulation of ammonium assimilation potential

Root glutamine synthetase (GS; EC 6.3.1.2) activity was measured daily (0 to 4 days) for eelgrass (Zostera marina L.) plants held under continuous darkness rooted in sediments, continuous darkness without sediments, continuous light without sediments, and control light/dark cycle (Control L/D). Roots experiencing prolonged aerobiosis exhibited lower activity in vitro than controls, whereas roots experiencing prolonged anoxia exhibited increased activity. Plants held in darkness without sediments had activity intermediate between controls and anoxic roots. One-hour pretreatment of root extracts with ATP slightly reduced in vitro glutamine synthetase activity, whereas pretreatment with ADP and AMP increased activity ≈50%. While glutamine synthetase activity increased with higher adenylate energy charge (AEC) in the reaction mixture, pretreatment of enzyme extracts at high adenylate energy charges decreased subsequent activity relative to pretreatment at lower energy charges. One-hour pretreatment with l-alanine (Ala) had little effect on enzyme activity. Pretreatment with l-glutamine (Gln), l-glutamate (Glu), and γ-amino butyric acid (GABA) increased activity ≈75%. Incubation of excised roots under anoxic conditions for 24 h nearly doubled enzyme activity. However, addition of cycloheximide to anoxic root incubations lessened or prevented the increase in activity. It appears that enhanced glutamine synthetase activity following periods of root anoxia results from interactions with metabolites that fluctuate between aerobic and anoxic conditions, particularly adenylates, and from de novo synthesis of glutamine synthetase or some other protein synthesis-dependent process.     

Nonnative protein polymers: structure, morphology, and relation to nucleation and growth

Thermally induced aggregates of α-chymotrypsinogen A and bovine granulocyte-colony stimulating factor in acidic solutions were characterized by a combination of static and dynamic light scattering, spectroscopy, transmission electron microscopy, and monomer loss kinetics. The resulting soluble, high-molecular weight aggregates (∼10³-10⁵ kDa) are linear, semiflexible polymer chains that do not appreciably associate with one another under the conditions at which they were formed, with classic power-law scaling of the radius of gyration and hydrodynamic radius with weight-average molecular weight (M_w). Aggregates in both systems are composed of nonnative monomers with elevated levels of β-sheet secondary structure, and bind thioflavine T. In general, the aggregate size distributions showed low polydispersity by light scattering. Together with the inverse scaling of M_w with protein concentration, the results clearly indicate that aggregation proceeds via nucleated (chain) polymerization. For α-chymotrypsinogen A, the scaling behavior is combined with the kinetics of aggregation to deduce separate values for the characteristic timescales for nucleation (τ_n) and growth (τ_g), as well as the stoichiometry of the nucleus (x). The analysis illustrates a general procedure to noninvasively and quantitatively determine τ_n, τ_g, and x for soluble (chain polymer) aggregates, as well as the relationship between τ_n/τ_g and aggregate M_w.     

Photosynthetic and morphological responses of eelgrass (Zostera marina L.) to a gradient of light conditions

Zostera marina L. (eelgrass) from Great Bay Estuary, New Hampshire and Maine (USA), was transplanted in outdoor mesocosms and subjected to four light treatments (100, 58, 34 and 11% surface irradiance, SI) between May and September 2003 to investigate the relationship between light availability and the growth and survival of eelgrass. Evaluating eelgrass seedlings and adult mature plants demonstrated no differences in photosynthetic response after 22 days of acclimation. During at least the first 19 days of shading, maximum electron transport rate (ETR_max) rate of eelgrass did not differ significantly between light treatments. After 40 days, a significant reduction in ETR_max and minimum saturating light was observed in plants growing at 34% SI and below. Morphological responses exhibited a linear increasing trend with greater light. 34% SI exhibited drastic reductions (to less than 25% of control) in rhizome growth, shoot density, shoot production, number of nodes per plant and plant weight at the end of the study (81 days). Shoot to root ratio at 34% SI increased by > 50%. Plants shaded to 58% SI showed no significant difference from the control in plant parameters except an increased rate of rhizome elongation. Our results link the lower shoot densities with shading to the slow growth rate of horizontal rhizomes and a total lack of lateral expansion at 11% SI. ETR_max declined over time in plants at 11% SI resulting in 81% mortality, no lateral branching and no morphological development, indicating that the minimum light required for long-term eelgrass growth and survival is greater than the previously suggested 11% SI. We demonstrate that eelgrass plants at these latitudes can persist at light levels of 58% SI and above, and are light-limited at 34% SI and below.