Depth Distribution of Thalassia testudinum in two Meadows on the West Coast of Florida; a Difference in Effect of Light Availability

Abstract. Shoot density was lower, while leaf area index and above- to below-ground biomass ratios were higher for Thalassia testudinum collected from the deep edge when compared to the shallow edge of a meadow off Anclotc Key, Florida. These three characters followed patterns predicted from previous studies. The characters were reversed in order for plants from shallow and deep edges of a meadow off Egmont Key, about 50 km south of Anclote Key. Causes other than light were not evident to explain this reversal in the Egmont Key meadow. This study suggests that although light limitation may be the usual limiting factor for depth distribution in seagrass meadows, other factors cannot be ignored in many meadows along the west coast of Florida.     

Experiments with Zooplankton on Coral Reefs,or, Will the Real Demersal Plankton Please Come Up?

Abstract. Zooplankton was sampled with emergence traps on a Thalassia bed behind an insular reef at San Salvador. Bahamas. These traps tested the effect of sealing the trap to the substrate or allowing it to rest unsealed on the substrate. Total numbers of plankters collected did not differ between the two types of trap, but significantly more cyclopoid copepods were collected in unsealed traps and significantly more tanaids were found in sealed traps. Total densities in unsealed traps were inflated as much as 24–28 % by contamination with holoplankton. Both trap designs, but particularly the sealed traps, collected animals that apparently crawled up the insides. A substantial number of animals, as many as 9–31% of those collected in sealed traps, may not be actually planktonic. Our results suggest that reefs produce fewer meroplankters per unit of substrate, and thus meroplankters contribute less to reef foodwebs than formerly thought.     

外源铁对高有机负荷下泰来草光合作用及抗氧化酶活性的影响

海草床是近岸重要的“蓝碳”生态系统,有机负荷升高导致的硫化物胁迫是海草床衰退的重要诱因;外源铁(Fe)的添加可有效改善高有机负荷沉积物中海草的生存状态。本研究在日光温室内,通过向两种不同类型(钙质和硅质)沉积物中添加淀粉(5 g/kg)模拟海草床高有机负荷,同时添加不同量的Fe(II)(0.00、0.05、0.15、0.50 g/kg),探究了泰来草(Thalassia hemprichii)叶片光合色素(Chl a、Chl b、Cx)含量、叶绿素荧光参数(F_m、F_v/F_m、F_v/F₀、Φ_PSII、ETR)及抗氧化酶(POD、SOD、CAT)活性对Fe添加的响应特点。结果表明,培养过程(30 d)中,两种类型的沉积物中外源Fe添加均能有效提升泰来草叶片光合色素含量、叶绿素荧光参数水平和抗氧化酶活性,且当Fe(Ⅱ)添加量为0.50 g/kg时效果最优。Fe添加量对间隙水Fe²⁺浓度、S^2-浓度、泰来草叶片光合色素含...     

海草凋落叶的溶解有机物的释放及其生物地球化学意义

Dissolved organic matter(DOM) represents a significant source of nutrients that supports the microbial-based food web in seagrass ecosystems. However, there is little information on how the various fractions of DOM from seagrass leaves contributed to the coastal biogeochemical cycles. To address this gap, we carried out a 30-day laboratory chamber experiment on tropical seagrasses Thalassia hemprichii and Enhalus acoroides. After 30 days of incubation, on average 22% carbon(C), 70% nitrogen(N) and 38% phosphorus(P) of these two species of seagrass leaf litter was released. The average leached dissolved organic carbon(DOC), dissolved organic nitrogen(DON) and dissolved organic phosphorus(DOP) of these two species of seagrass leaf litter accounted for 55%, 95% and 65% of the total C, N and P lost, respectively. In the absence of microbes, about 75% of the total amount of DOC, monosaccharides(MCHO), DON and DOP were quickly released via leaching from both seagrass species in the first 9 days. Subsequently, little DOM was released during the remainder of the experiment. The leaching rates of DOC, DON and DOP were approximately 110, 40 and 0.70 μmol/(g·d). Leaching rates of DOM were attributed to the nonstructural carbohydrates and other labile organic matter within the seagrass leaf. Thalassia hemprichii leached more DOC, DOP and MCHO than E. acoroides. In contrast, E. acoroides leached higher concentrations of DON than T. hemprichii, with the overall leachate also having a higher DON: DOP ratio. These results indicate that there is an overall higher amount of DOM leachate from T. hemprichii than that of E. acoroides that is available to the seagrass ecosystem. According to the logarithmic model for DOM release and the in situ leaf litter production(the Xincun Bay, South China Sea), the seagrass leaf litter of these two seagrass species could release approximately 4×10~3 mol/d DOC, 1.4×10~3 mol/d DON and 25 mol/d DOP into the seawater. In addition to providing readily available nutrients for the microbial food web, the remaining particulate organic matter(POM)from the litter would also enter microbial remineralization processes. What is not remineralized from either DOM or POM fractions has potential to contribute to the permanent carbon stocks.     

Physiological responses of the seagrass Thalassia hemprichii (Ehrenb.) Aschers as indicators of nutrient loading

To select appropriate bioindicators for the evaluation of the influence of nutrients from human activities in a Thalassia hemprichii meadow, environmental variables and plant performance parameters were measured in Xincun Bay, Hainan Island, South China. Nutrient concentrations in the bay decreased along a gradient from west to southeast. Moreover, the nutrients decreased with an increase in the distance from the shore on the southern side of the bay. Among the candidate indicators, the P content of the tissues closely mirrored the two nutrient loading gradients. The epiphytic algae biomass and the N content in the tissues mirrored one of the two nutrient loading trends. The leaf length, however, exhibited a significant negative correlation with the nutrient gradients. We propose that changes in the P content of T. hemprichii, followed by epiphytic algae biomass and N content of the tissues, may be the useful indicators of nutrient loading to coastal ecosystems.     

Epiphyte loads on seagrasses and microphytobenthos abundance are not reliable indicators of nutrient availability in oligotrophic coastal ecosystems

Despite marked gradients in nutrient availability that control the abundance and species composition of seagrasses in south Florida, and the importance of nutrient availability in controlling abundance and composition of epiphytes on seagrasses in other locations, we did not find that epiphyte load on the dominant seagrass, Thalassia testudinum, or that the relative contribution of algal epiphytes to the epiphyte community, was positively correlated with nutrient availability in the water column or the sediment in oligotrophic seagrass beds. Further, the abundance of microphytobenthos, as indicated by Chlorophyll-a concentration in the sediments, was not directly correlated with concentrations of nutrients in the sediments. Our results suggest that epiphyte and microphytobenthos abundance are not unambiguous indicators of nutrient availability in relatively pristine seagrass environments, and therefore would make poor candidates for indicators of the status and trends of seagrass ecosystems in relatively low-nutrient environments like the Florida Keys.