Field transplantation of seagrass (Posidonia oceanica) seedlings: Effects of invasive algae and nutrients

The positive impact of conservation action

Governments recently adopted new global targets to halt and reverse the loss of biodiversity. It is therefore crucial to understand the outcomes of conservation actions. We conducted a global meta-analysis of 186 studies (including 665 trials) that measured biodiversity over time and compared outcomes under conservation action with a suitable counterfactual of no action. We find that in two-thirds of cases, conservation either improved the state of biodiversity or at least slowed declines. Specifically, we find that interventions targeted at species and ecosystems, such as invasive species control, habitat loss reduction and restoration, protected areas, and sustainable management, are highly effective and have large effect sizes. This provides the strongest evidence to date that conservation actions are successful but require transformational scaling up to meet global targets.

Use of a wastewater recovery product (struvite) to enhance subtropical seagrass restoration

Seagrasses are in decline worldwide, and their restoration is relatively expensive and unsuccessful compared to other coastal systems. Fertilization can improve seagrass growth in restoration but can also release nutrients and pollute the surrounding ecosystem. A slow-release fertilizer may reduce excessive nutrient discharge while still providing resources to the seagrass's rhizosphere. In this study, struvite (magnesium ammonium phosphate), a relatively insoluble, sustainable compound harvested in wastewater treatment plants, was compared to Osmocote™(14:14:14 Nitrogen: Phosphorus: Potassium, N:P:K), a popular polymer coated controlled release fertilizer commonly used in seagrass restoration. Two experiments compared the effectiveness of both fertilizers in a subtropical flow-through mesocosm setup. In the first experiment, single 0.5 mg of P per g dry weight (DW) doses of Osmocote™and struvite fertilizers were added to seagrass plots. Seagrass shoot counts were significantly higher in plots fertilized with struvite than both the Osmocote™and unfertilized controls (p< 0.0001). A significant difference in total P concentration was observed in porewater samples of Osmocote™vs struvite and controls (p< 0.0001), with struvite fertilized plots emitting more than controls (p ≤ 0.0001), but less than 2% of the total dissolved P (TDP) of Osmocote™fertilized plots (100+ mg/L versus x > 5 mg/L). A subsequent experiment, using smaller doses (0.01 and 0.025 mg of P per gram DW added), also found that the struvite treatments performed better than Osmocote™, with 16-114% more aboveground biomass (10-60% higher total biomass) while releasing less N and P. These results indicate the relatively rapid dissolution of Osmocote™may pose problems to restoration efforts, especially in concentrated doses and possibly leading to seagrass stress. In contrast, struvite may function as a slow-release fertilizer applicable in seagrass and other coastal restoration efforts.

An examination of seed germination and seedling growth of Zostera marina for planting-time selection in Rongcheng Bay, Shandong Peninsula, China

This study firstly quantified the responses of seeds of Zostera marina to different planting times (22 September, 5 October, 23 October, 7 November and 20 November in 2015) through a field seed-planting experiment over a two year period. The suitable seed planting time required by the seeds of Z. marina was evaluated. The seedling establishment rate of Z. marina subjected to different planting times ranged from 7% to 55%, with the higher values attained on the treatments of 22 September and 5 October. New plant patches from seed were fully developed and well maintained on the planting time of 22 September, 5 October and 23 October after 2 years following planting. The shoot density under the three treatments ranged from 62 shoots per replicate to 72 shoots per replicate with an average of 67 shoots per replicate in September 2017. According to the propagation assessment and growth analysis, we found that the planting time from mid-September to mid-October may be the optimal time to plant seeds of Z. marina in our experimental site. Our results demonstrate that seed planting time has an important effect on the effectiveness of eelgrass restoration and provide data that could prove helpful in the development of successful eelgrass restoration.

Poor success of seagrass Posidonia oceanica transplanting in a meadow disturbed by power line burial

Local disturbances drive the decrease of the area covered by Posidonia oceanica in the Mediterranean. Mechanical impacts during the development of coastal infrastructures alter sea floor and the recolonization of benthic community will depend on the recovery of pre-disturbance environmental conditions and on the intrinsic characteristics of the local community that was disturbed. We transplanted 468 rhizome fragments and 450 seedlings of P. oceanica in a meadow disturbed by the trenching and deployment of a power line to evaluate the suitability of the disturbed sea floor for rehabilitating P. oceanica meadows. We quantify and compare the survivorship and vegetative development of the transplanted/planted (i.e. fragments/seedlings) material in the two types of the unconsolidated substrata left after infrastructure deployment works finished: sand and burlap bags filled with coarse gravel. The latter was used as a corrective measure for topographic restoration. Three experimental plots with sixteen transplanted fragments or twenty-five seedlings were placed at each substratum type at three different depths (i.e. 15, 20 and 25 m). Our results show that the transplanting of P. oceanica rhizome fragments in the disturbed substrata had low survival rates (0-31%) after 40-48 months. The survivorship of seedlings was lower than that of fragments. Our results highlight the importance of substratum for P. oceanica recovery after mechanical impact; disturbed, non-consolidated substrata will preclude P. oceanica rehabilitation through planting. Preservation of meadow substratum (i.e. dead matte) is a critical element that coastal infrastructure projects should consider to enable future recovery of P. oceanica meadows.

Effects of interactions between macroalgae and seagrass on the distribution of macrobenthic invertebrate communities at the Yellow River Estuary, China

Algae-dominance in seagrass beds has been well recognized, however, the competitive relationship between seagrass and macroalgae along land-sea gradients and their ecological effects has received little attention. In this study, a field survey was conducted at the Yellow River Estuary to investigate the effects of macroalgal proliferation on seagrass and macrobenthic invertebrate communities. Our results suggested that strong competitive interaction existed between the two primary producers, and the positive or negative effects of macroalgae on seagrass growth varied along land-sea gradient. Furthermore, the dominant controlling factors on the biomass, density and diversity of macrobenthic invertebrate communities were found to vary accordingly, i.e., from features of the primary producers in the nearshore where macroalgae suppressed seagrass growth to hydrodynamic disturbance in the offshore where macroalgae facilitated seagrass growth. Our study emphasizes the importance to integrate interspecific competition into ecosystem-based management of seagrass ecosystem, and provides references for additional ecological indicators.

Influence of biotic and abiotic factors of seagrass Posidonia oceanica recruitment: Identifying suitable microsites

The period between seed germination and successful seedling establishment is considered the most vulnerable phase for plant development. To better predict recruitment patterns within plant communities, it is essential to identify the abiotic constrains and biotic interactions that allow for the colonization of substrates by plant species. We evaluated which combination of factors are associated with successful survival and development of seedlings of the seagrass Posidonia oceanica in order to identify the most important microsite features acting together on recruitment success. Our results show that P. oceanica seedlings are rather specific in their environmental requirements during their first 18 months of life, when their development and survival are favored in microsites of consolidated substratum (solid rock, and to a lesser extent P. oceanica matte) covered by macroalgae (mainly crustose algae) and located in sheltered locations (with energy flux values not exceeding 7 × 10⁵ kg s^-2 m s^-1). After this phase, their probability of surviving becomes more independent from external conditions.

Heavy metal accumulation and phytoremediation potential by transplants of the seagrass Zostera marina in the polluted bay systems

Although seagrasses can incorporate heavy metals from the marine environment, few studies have been conducted on heavy metal uptake and phytoremediation potential by seagrass transplants in the heavy metal contaminated sediments. Zostera marina shoots were transplanted in two polluted bay systems on Korean coasts to evaluate the heavy metal contaminations in sediments and the possibility of using Z. marina transplants as a bioindicator and phytoremediation agent. The major concentrated metals in sediments were As, Cu, Fe, and Pb in Jaran Bay, and Cd, Co, Zn, and Hg in Onsan Bay. The Co, Zn, Pb, and Hg concentrations in Z. marina tissues reflected the sediment heavy metal concentrations, and thus the tissue heavy metal concentrations may be used as bio-indicators of the metal contaminations. Since Z. marina transplants accumulated a great amount of heavy metals in their tissues, they may have the phytoremediation potential for the heavy metal contaminated sediments.