Dedicated Section: The Fifth International Seagrass Biology Workshop, 7-11 October 2002, Ensenada, Baja California, Mexico
Gulf of Mexico Science
explains the observed age-dependent Fv/Fm reduction. The variation in the photochemical signal registered along seagrass leaves integrates light environmental pressure as well as self-shading effects within the seagrass canopy. In this work, I examine the potential of studying this permanent Fv/Fm reduction to understand seagrass photoacclimation. Comparisons have been done to test differences in the pattern of Fv/Fm variation: (1) between leaves of two different species, Zostera marina and T.
... estudinmn, growing under very contrasting light environments such as a Danish estuary and a coral reef lagoon in the Mexican Caribbean; (2) among T. testudinwn leaves growing in six different meadows at different depths in a Puerto Morelos coral reef lagoon, strongly differing in their biomass density and in the light attenuation coefficient (Kd) within the seagrass canopy; and (3) among T. testudinmn leaves growing within the same meadow but supporting different flowering effort. These comparisons highlight the potential of the permanent Fv/Fm reduction along seagrass leaves as a powerful tool to comprehend seagrass performance. Zostn·a marina is the most abundant submerged aquatic macrophyte from coastal lagoons in Baja California, Mexico. In this study, we evaluated annual fluctuations of photosynthetic characteristics and biochemical composition of Z. mmina inhabiting northern and southern lagoons of Baja California. In situ irradiance and water temperature were monitored throughout the year at San Quintin (northern lagoon) and at Ojo de Liebre and San Ignacio (southern lagoons). In situ irradiance was approximately twofold greater and water temperature was 5 C greater at the southern lagoons than at the northern one. Chlorophyll levels in the tissue of Z. marina were twofold greater at San Quintin, suggesting an acclimation to the lower irradiance levels at the northern lagoon than at the southern ones. Photosynthetic characteristics were relatively similar in shoots from all three sites. The hours of light saturated photosynthesis (Hsat), calculated from the photosynthetic characteristics and irradiance levels, suggest that Z. marina is light-limited during 20% of the year at the northern lagoon. Carbohydrate levels were twofold greater at the northern lagoon, suggesting that chlorophyll levels compensate for the lower light levels at this site relative to the southern lagoons. In contrast to carbohydrate levels, total protein levels were approximately 50% greater at the southern lagoons than the northern one. Fiber content and calories in the leaves of Z. marina were similar at all three sites. The distribution of Z. mmina in the northern lagoon is approximately 1 m higher in the intertidal zone than meadows at the southern lagoons, suggesting that these populations are colonizing shallower areas as a response to lower light levels. In a 1-yr in situ shading experiment, carbon allocation was studied in four different experimental events. Two contrasting seagrass species were selected for the study: the short-lived, early successional Ha1odule uninervis and long-lived, late successional Thalassia hemp1ichii. The stable isotope 13 C (NaH 13 C0 3 ) was used to quantify carbon allocation to the different plant parts. Natural 13 C/ 12 C was lower in H. uninavis (-9.5 to -10.2 %o) than in T. hemprichii (-5.9 to -6.6 %o). The 8 13 C values of the different plant parts were significantly different in both species, the leaves being more 13 C-depleted than the roots and the rhizomes. In both species, tracer addition resulted in a significant enrichment compared to 13 C natural abundance, in H. uninervis the enrichment representing twofold than in T. hemprichii. Shading up to 89% light reduction did not influence the enrichment in either species. However, significant differences were observed between experimental events or sampling periods and between species. The amount of 13 C remaining in the plant 5 d after the 3-hr incubations was less