Underwater Vegetation, Aquatic Cultivation
- UNDERWATER VEGETATION/UNDERWATER PLANTS
- Environmental Benefits of Underwater Vegetation
- Chemical Pollution
- Biological Pollution
- Physical Pollution
- Hydrodynamic Alterations of Shipping
- Scuba-Diving Tourism
- Marine Structures
- Global Warming
- Underwater Crop Protection and Restoration
Zahra Naghibi, Jacqueline A. Stagner,
Rupp Carriveau, and David S.K. Ting
The gap between growing populations and food supply, limited resources in terms of available w'ater and arable land, climate change, soil degradation, urban sprawl, and a lack of distribution equity contribute to the challenge of meeting the planet’s food demand . Considering this increasing food demand, taking advantage of all food resources is crucial. Water ecosystems are a significant source of human food. Water, in the form of fresh water and seawater, covers more than 70% of the Earth’s surface. Saving the underwater natural ecosystems plays an important role in saving the planet. Underwater vegetation or submerged aquatic vegetation (SAV), as one of the key components of aquatic ecosystems and food chains, has a crucial role in maintaining the biodiversity and overall health of ecosystems  and supplying food for humans.
Over recent decades, many underwater vegetation have been reported to be declining as a result of human activities . It is crucial to understand the effect of human activities on aquatic vegetation to manage and protect aquatic environments. In the first part of this section, the environmental benefits of underwater vegetation, a summary of harmful activities and pollution that threatens the health of underwater plants, and some protection and restoration methods will be discussed.
Transplanting of underwater vegetation is one of the methods used for aquatic restoration. Transplanting is a type of cultivation. However, aquatic cultivation is not restricted to transplanting to restore natural ecosystems. Aquatic cultivation can be used to produce food for human consumption. Agricultural water shortages, rising sea levels as a result of global warming, and soil disappearing are some reasons for thinking about other agricultural methods. In this chapter, floating agriculture with a focus on seaweed cultivation and soilless platforms, floating greenhouses using ships, and very large floating structures (VLFSs) will be mentioned as climate change adaptation methods that are responses to the global food demand.
Another important food resource for humans is fish. Underwater vegetation has positive interactions with fishes. Many aquatic plants provide a nursery or habitat area for fish assemblages in part or all of their life cycles to protect them from predators. In addition, some underwater vegetation can serve as the food for herbivorous and omnivorous fishes. On the other hand, fish can have an important ecological role in protecting underwater vegetation. In the final part of this chapter, this positive fish-underwater vegetation interaction will be discussed.
UNDERWATER VEGETATION/UNDERWATER PLANTS
Underwater crops, as one of the main components of aquatic environments, have many environmental benefits. They can be a link in the food chain or a cover for small fish and invertebrates. In recent decades, many underwater plant species are reported to be declining worldwide  as a result of global warming, various kinds of pollution, and physical trauma as the consequence of shipping and scuba diving. Underwater vegetation must be protected locally and globally to maintain important ecosystem functions and structures. In this section, some environmental benefits of underwater vegetation will be discussed. Next, a summary of activities that threaten aquatic crops will be described. Finally, some methods of underwater crop protection and restoration will be discussed.
Environmental Benefits of Underwater Vegetation
Underwater vegetation has important environmental benefits. It can act as the base of herbivorous and detrivorous food chains  such as invertebrates, fish, birds, and mammals during one or more stages of their life cycle. Underwater vegetation can also be a source of organic carbon for bacteria [4,5]. In addition, some aquatic plants can provide shelter and nursery areas for prey . In other words, vegetation can provide a habitat for ecological communities or ecosystems.
In addition, underwater crops can be considered the earth’s carbon sink . During photosynthesis, aquatic crops consume carbon dioxide (CO,) in the water and produce oxygen. On the other hand, during respiration at night, they consume dissolved oxygen and produce CO,. Recently, research on ponds has showm that under the optimum photosynthesis conditions, aquatic crops have potentially significant role in stabilizing the carbonate weathering-related carbon sink . In the other words, there is a net C02 sink directly from the atmosphere to the water body. Therefore, underwater vegetation can significantly affect the terrestrial carbon budget, in some cases .
Storms, tsunami waves, and tropical cyclones can cause considerable damage along shorelines . Aquatic crops, by buffering the wave energy and reducing flow velocity, can protect shorelines from storms and sediment erosion . Wetland habitats, such as mangroves and salt marshes, can provide wave-energy and storm-surge buffers [10,11]. By reducing the bed shear stress, they are also able to trap and stabilize sediment and reduce regional erosion .
Some aquatic plants act as marginal filters that protect lands from the migration of contaminants and sediments. They act as a barrier for the migration of contaminants in the transition zone that connects the land and sea . Contaminants could be organic pollutants, heavy metals, and pathogenic microorganisms . Aquatic vegetation can also enhance water clarity by trapping sediment .
Although underwater vegetation has many environmental benefits, over the past several decades, many aquatic vegetables in different water habitats have experienced abundant decline around the world . Any reduction in aquatic vegetation can result in habitat degradation, which in turn can cause a reduction in ecosystem service functions . The origin of this decline is directly or indirectly because of human activities . The first key in protecting aquatic ecosystems is understanding the effect of human activities. The following is a summary of the activities that act as a threat to aquatic crops.
Some human activities can input toxic substances into aquatic ecosystems . This chemical pollution reduces the water quality or is accumulated in the sediments . The sources of chemical pollution could be from outboard engines of vessels, fuel spillage, sewage, and agricultural fertilizers . For example, acidification of lakes affects the accessibility of inorganic nutrients and carbon for macrophytes . In addition, the concentration of some metal ions can change the underwater light availability .
Excessive growth of invasive alien aquatic plants (IAAPs) in marine benthic ecosystems can result in biological pollution . IAAPs are considered a serious threat to global biodiversity  because some of these species are hazardous and compete with native plants to gain light, space, and nutrients . In some cases, loss of native aquatic plants can happen as a result of invasive aquatic plants . One of the main routes of this biological invasion is commercial shipping and water vessels ; the vessels act as a vector for invasion .
Physical pollution of water is caused by suspended solids and is a global danger to freshwater crops. For example, a massive influx of drifting Sargassum, which consists of Sargassum fluitans and Sargassum natans, in the Caribbean Sea between 2011 and 2016 was experienced . These species have positive effects on ocean ecosystems; however, the beaching of them along the coastline resulted in a buildup of brown-colored, beach-cast material. Tussenbroek et al.  used the term “Sargassum-brown-tide” (Sbt) for this phenomena. In Figure 3.1, the effect of Sbt on the seagrass of three sites has been shown. They evaluated the effect of Sbt on light, oxygen, and pH, for four near-shore water locations.
FIGURE 3.1 Seagrass meadows before and after Sargassum “brown tide” at three different sites. (Reproduced with permission from the literature van Tussenbroek, В. I. et al.. Mar. Pollut. Bull., 122. 272-281, 2017.)
Another example of physical pollution is sediment burial. Sediment loading can cause deterioration in the water quality and underwater light, which is critical for aquatic crops . In ecosystems consisting of seagrass and algae, the condition is more complicated. It has been shown that witli increasing nutrient and sediment load, the seagrass system will shift toward an algae-dominated system at some point, which could be an irreversible state transition in some cases .
Hydrodynamic Alterations of Shipping
Shipping creates hydrodynamic alterations that threaten aquatic ecosystems. Impacts of ship-induced waves on underwater vegetation can be separated into two groups : (i) impacts on the underwater vegetation growth as the result of damage and uprooting and (ii) changing species composition, distribution, and abundance. Doyle  experimentally studied the effect of 0.15-m waves on young Vallisneria americana plants. It was shown that the total mass accumulation of plants exposed to the disturbed water was 50% of the undisturbed plants. In addition, the disturbed ones had shorter leaves.
Scuba-diving tourism has increased significantly in recent decades . At the same time, the environmental interaction of scuba divers and underwater ecosystems, and environmental issues regarding this industry, have attracted considerable research interest [30,31]. Lucrezi et al.  summarized the negative environmental impacts of scuba diving, including direct damage to undewater ecosystems through physical contact with the habitats and indirect issues of scuba diving because of the development of coastal zones. For instance, coral reefs, destinations for many recreational divers, have experienced a significant decline over recent decades as a result of physical contact from divers .
The increasing number of marine and coastal structures in water environments is causing “ocean sprawl” . Ocean sprawl can have a crucial impact on natural ecosystems. Structures built over underwater vegetation may shade and make a physical barrier for the light and, as a result, kill the macrophytes . These impacts are not only restricted to the location of the structures but can also have a critical impact on the regional scale . As an example, construction of low-crested structures to defend shores from erosion have some local impacts, such as disruption of surrounding soft-bottom environments, and regional impacts on regional species diversity and favoring the spread of non-native species .
As a result of global warming, world water temperatures have increased significantly in recent decades. According to the Intergovernmental Panel on Climate Change (IPCC), sea surface temperatures have also seen a rise of 0.4°C-0.8°C in the past century . Water temperature rise can have impacts on individual, plant-herbivore interactions, or native plant-invasive aquatic plant interactions.
In recent decades, many species of underwater vegetation have experienced a rapid decline as a result of increased water temperatures. For example, coral reef cover has experienced a rapid decline due to increased water temperatures across the Caribbean region . In addition, rising water temperatures influence the interactions between species. Pages et al.  showed that as the result of changing the complex factors determining species interaction in the warming Mediterranean, there will be a clear set of winners and losers. The response of selected plant and animal interactions are shown in Figure 3.2. Their proposed model can evaluate the effect of warming on the interaction of the plants (Posidonia oceanica seagrass, seagrass Cymodocea nodosa, and macroalga Cystoseira mediterranea) and herbivores (sea urchins).
To investigate the effect of global warming on native plants and invasive alien plants, Verlinden et al.  considered two pairs of Senecio inaequidens—Plantago lanceolata (alien invasive—native) with the domination of the alien species and Solidago gigantea—Epilobium hirsutum with the domination of the native species. The results showed that rising temperatures caused invasion-dominance reduction in the first pair, and the dominance of the invader increased in the second pair. According to their study, the effect of warming on invasive plants must be considered with their interaction with native plants. Monoculture behavior is not a sufficient indicator of plant response to climate change.
FIGURE 3.2 The outcomes of plant-herbivore interactions in a warming Mediterranean. Arrows show the direction of warming, (a) The herbivore pressure on Posidonia oceanica seagrass remained unchanged with warming; however, the decrease in herbivore happened when warming increased because with rising temperature, the P. Oceanica seagrass leaves incubated. (b) The herbivore pressure decreased for seagrass Cymodocea nodosa during warming, (c) The herbivore pressure increased for macroalga Cystoseira mediterranea during warming because of the low performance of the macroalga at warm temperatures. (Reproduced with permission from the literature Pages, J.F. et al.. Mar. Pollut. Bull., 134, 55-65, 2018.)
Underwater Crop Protection and Restoration
The global decline in the underwater vegetation demands urgent management strategies to protect them. Consideration of marine-protected areas (MPAs), land-use patterns, and transplantation are some important methods of underwater crop protection or restoration that will be discussed in this section. These methods can prevent the harmful human activities motioned in the previous section and have an important role in the restoration of the threatened area.
The creation of MPAs is an important management tool to reduce the trend of biodiversity loss in marine ecosystems . The International Union for Conservation of Nature (IUCN) defined the term “marine protected area” as “any area of intertidal or subtidal terrain, together with its overlying waters and associated flora, fauna, historical and cultural features, which has been reserved by legislation to protect part or all of the enclosed environment” . According to the IUCN, these areas should protect against outside activities, and depleted, threatened, rare, or endangered species should be protected in these areas . According to the United Nations Environment Programme (UNEP), there has been considerable grow'th in MPAs across the w'orld in recent years . The current percentages of MPAs are shown in Figure 3.3. In July 2018,45.7% of nearshore marine ecoregions have been met the marine protection target of 10%.
Quiros et al.  showed that land use can be more important than marine protection in some cases. In their study, they considered the relationship between terrestrial and marine systems to evaluate the effects of human land-based activities on seagrasses. They found that human activities on land have major impacts on the abundance and richness of seagrass. They saw this pattern across a large spatial scale (more than 800 km) and more than 54 samples of seagrass meadow'.
FIGURE 3.3 Percentage of protected marine areas coverage. (Reproduced with permission from the literature UNEP-WCMC. IUCN, and NGS. “Protected Planet Report 2018.” UNEP-WCMC. IUCN and NGS: Cambridge, UK; Gland, Switzerland; and Washington, DC, USA, 2018.)
The management of the integration of terrestrial and aquatic environments has been proven in other studies, as well. Employment of concurrent aquatic and terrestrial conservation actions is delivering cost-effective outcomes for marine conservation in comparison with other land- or sea-based conservation actions to protect aquatic ecosystems .
In addition to the management and conservation of aquatic vegetables, restoration actions are needed in marked habitat areas with degradation and reduction of aquatic vegetation across the world . Transplantation of some aquatic vegetation is recognized as the best method for their restoration. For example, it is a conventional method in the restoration of coral reefs. Jaap (2000)  summarized steps for the transplantation of physically damaged coral reefs. These steps include eliminating loose debris from the reef, rebuilding three-dimensional structures into leveled- scarified reef surfaces, and transplanting reefs back. However, some deficiencies have been mentioned for the transplanting method. Four of them were mentioned by Li et al. (2008) . First, low efficiency and being labor-intensive limit its application for large scales. Second, this method usually causes great harm to the underwater vegetation at the source site. Third, transplanting can be considered a “legal” way of spreading invasive aquatic plants, and because of that, in some cases finding native macrophytes has become more difficult. Fourth, the plants restored with the transplanting are not stable.
Monitoring the impacts of human activities on underwater health conditions helps to understand the interaction of human and underwater vegetation and improve the management restoration and protection tools by the time. In this way, education also plays an important role in underwater crop protection. The more humans learn about the underwater ecosystem threats and ways of protection, the better prepared they will be to make deep and lasting changes. All methods of protection and restoration have their advantages and disadvantages, and the selection of the best method depends on the type of crop which needs to be restored. Depending on the environmental issue that aquatic plants are struggling with and the type of underwater vegetation, different approaches or methods will be adopted to restore the aquatic plants.