• Energy Research

In this study, carried out in San Antonio Bay (Northern Argentinean Patagonia), we aimed to understand the relative importance of bottom-up and top-down controls on macroalgal blooms in a macrotidal system with high nutrient supply and high consumer abundance. Our results show that nutrients, pH, and O 2 concentrations were higher during low tide. A field experiment showed that the biomass accumulation rate of Ulva lactuca ranged from 6 to 12% d -1 and was reduced by herbivory by 60%. The biomass accumulation rate did not differ in thalli with different initial internal nutrient pools. There was a negative relationship between the percentage of algae consumed and the N content in algal tissues, suggesting compensatory feeding by herbivores. Herbivory reduced the biomass accumulation rate of U. lactuca when PO 4 3- or no nutrients were added, but not when NO 3 - was added. In the absence of herbivory, the addition of nutrients did not increase U. lactuca biomass accumulation rate. These results suggest that nutrients remain high enough for adequate time intervals to be assimilated by macroalgae and support blooms. Large water exchange during tidal changes, however, can diminish the potential negative effects of macroalgal accumulation (oxygen depletion, high ammonium concentrations) on herbivores such that herbivores can have a large impact on macroalgae. © Inter-Research 2011.

Nitrogen supply is a major control on growth of coastal macroalgae (1, 2, 3). Top-down effects in which grazing signifi cantly affects macroalgae (4, 5), and nutrient-grazer interactions (3) have also been described. In this paper we describe an experiment in which we measured net growth of a common macroalga, Ulva lactuca, in treatments that allowed different numbers of grazers to access fronds as well as incubation of fronds in estuaries with demonstrably different nutrient supplies. These treatments were intended to assess the relative infl uence of grazer and nitrogen supply on net growth rates of a coastal producer. To examine the effect of grazing on growth of U. lactuca, we constructed acrylic plastic cages with sides of 1-mm, 4-mm, or 18-mm mesh. The different mesh openings were intended to allow entry to different numbers of grazers, which we took as a proxy for grazing pressure. The cage design also allowed for light penetration and horizontal water fl ow. The 18-mm mesh permitted larger size classes and a greater number of grazers to enter the cages, while the 1-mm mesh excluded larger size classes and allowed fewer grazers. The 4-mm mesh was intended to furnish an intermediate grazer treatment. To evaluate the effect of nitrogen supply and grazing on algal growth, cages with the three mesh sides were placed in three estuaries in Waquoit Bay, Massachusetts. These three estuaries experience different nitrogen loads—Sage Lot Pond, 14 kg ha 1 y 1 ; Quashnet River, 350 kg ha 1 y 1 ; and Childs River, 601 kg ha 1 y 1 —from their watershed (6). These nitrogen loads led to different mean nitrate concentrations measured in the estuaries during July 2002, one year prior to the time of our experiments: 0.04, 6.1, and 11.75 M for Sage Lot Pond, Quashnet River, and Childs River, respectively (G. Tomasky, Boston University Marine Program, unpubl. data). To minimize effects of differences between estuaries other than our treatments, we chose sites similar in salinity, depth, and algal composition. In each estuary we placed four replicates of each of the three grazing pressure treatments, for a total of 36 cages. Three fronds of U. lactuca, each approximately 300 mg (blotted wet weight), were suspended inside each cage. To measure the effect of top-down versus bottom-up factors, we measured net growth as the dependent variable. Net growth was the growth achieved by the fronds minus the biomass consumed by grazers. To determine net growth, the U. lactuca fronds were weighed initially (blotted wet weight) and again after 10 days of fi eld incubation. First, we assess the successed of the treatments. To roughly measure the grazing pressure, we sorted and counted the potential grazers found in the cages for two replicates of each treatment at the end of the incubation. The grazers were sorted into four groups, amphipods, shrimp, crabs, and isopods. The total number of grazers in the 1-mm mesh cages was signifi cantly lower in all three estuaries than the number in the 4-mm cages (Fig. 1a; ANOVA F 31.0, P 0.0014). The number of grazers found in the 18-mm mesh cages was also signifi cantly different, although they contained lower grazer abundances than the 1-mm and 4-mm mesh cages (Fig. 1a). Predatory fi sh and large shrimp entered the cages with 18 mm mesh and likely fed on the smaller grazers, thus decreasing grazer abundances. This possible effect of predators on grazers suggests that there might be important top-down cascade effects in this system waiting to be studied. The difference in nitrogen load in the three estuaries provided quantitatively different nutrient supplies, as evident in the nitrate concentrations cited above. Bottom-up effects from these different nitrogen supplies on net growth of U. lactuca were dominant factors. Rates of net growth were higher in estuaries receiving

We studied if functional traits related to resource preemption (light and inorganic nutrients) exert control on space preemption of tropical seagrass meadows. Additionally, we studied if space preemption changed under different eutrophication scenarios. We took seagrass abundance data to study space preemption, seagrass traits data to study their effect on space preemption and eutrophication indicators to evaluate the level of eutrophication at each site/sampling event. The data was collected in Unguja Island (Zanzibar Archipealgo, Tanzania) in seven sites/sampling events (Harbor, Chapwani, Changuu, Bweleo, Fumba, Mangroves and Marumbi). Each site/sampling event comprised a subtidal seagrass meadow (2-4 meters depth) of around 2500 square meters, delimited by the coastline and a fringing reef. The data was taken between the 26.09.2016 to the 05.10.2016. In each site/sampling event, five 50 meters transects were deployed perpendicular to the coast and paralel to each other, approximately separated by 50 meters. The areas enclosed beweeen the transects were names A, B, C and D. Macroalgae biomass was collected as an indicator of eutrophication. Macroalgae biomass was quantified along five 50-m transects per site/sampling event, set perpendicular to the coast and parallel to each other, separated by ~50 meters. We collected the macroalgae present in three random 0.25x0.25 meters quadrats per transect. The macroalgae samples were cleaned of sediments and rinsed with water. They were then dried at 50°C in a forced air oven until constant dry weight. The macroalgae biomass was calculated as the grams of dry weight divided by the area of the quadrat (grams of dry weight per square meter).

SI

Seagrass meadows provide valuable ecosystem services but are fragile and threatened ecosystems all over the world. This review highlights the current advances in seagrass research from Viet Nam. One goal is to support decision makers in developing science-based conservation strategies. In recent years, several techniques were applied to estimate the size of seagrass meadows. Independent from the method used, there is an alarming decline in the seagrass area in almost all parts of Viet Nam. Since 1990, a decline of 46.5% or 13,549 ha was found. Only in a few protected and difficult-to-reach areas was an increase observed. Conditions at those sites could be investigated in more detail to make suggestions for conservation and recovery of seagrass meadows. Due to their lifestyle and morphology, seagrasses take up compounds from their environment easily. Phytoremediation processes of Thalassia hemprichii and Enhalus acoroides are described exemplarily. High accumulation of heavy metals dependent on their concentration in the environment in different organs can be observed. On the one hand, seagrasses play a role in phytoremediation processes in polluted areas; on the other hand, they might suffer at high concentrations, and pollution will contribute to their overall decline. Compared with the neighboring countries, the total Corg stock from seagrass beds in Viet Nam was much lower than in the Philippines and Indonesia but higher than that of Malaysia and Myanmar. Due to an exceptionally long latitudinal coastline of 3,260 km covering cool to warm water environments, the seagrass species composition in Viet Nam shows a high diversity and a high plasticity within species boundaries. This leads to challenges in taxonomic issues, especially with the Halophila genus, which can be better deduced from genetic diversity/population structures of members of Hydrocharitaceae. Finally, the current seagrass conservation and management efforts in Viet Nam are presented and discussed. Only decisions based on the interdisciplinary cooperation of scientists from all disciplines mentioned will finally lead to conserve this valuable ecosystem for mankind and biodiversity.

AbstractSeaweed and seagrass communities in the northeast Atlantic have been profoundly impacted by humans, and the rate of change is accelerating rapidly due to runaway CO2 emissions and mounting pressures on coastlines associated with human population growth and increased consumption of finite resources. Here, we predict how rapid warming and acidification are likely to affect benthic flora and coastal ecosystems of the northeast Atlantic in this century, based on global evidence from the literature as interpreted by the collective knowledge of the authorship. We predict that warming will kill off kelp forests in the south and that ocean acidification will remove maerl habitat in the north. Seagrasses will proliferate, and associated epiphytes switch from calcified algae to diatoms and filamentous species. Invasive species will thrive in niches liberated by loss of native species and spread via exponential development of artificial marine structures. Combined impacts of seawater warming, ocean acidification, and increased storminess may replace structurally diverse seaweed canopies, with associated calcified and noncalcified flora, with simple habitats dominated by noncalcified, turf‐forming seaweeds.