The seagrass commonly referred to as ‘Eelgrass’ (Zostera marina) is one of two ‘true’ seagrass species found among UK coastal regions and is currently threatened with rapid environmental changes from anthropogenic and natural stressors. Due to its high light requirement, Z. marina has a maximum growth restriction of approximately 7m and as a result is easily susceptible to damage and fragmentation from factors such as anchoring, therefore the ecological services which they provide are also at risk. Fragmentation can be described as larger patches breaking down into smaller patches, whilst decreasing habitat quality and a loss of resource for associated organisms. Z.marina provides a fundamental nursery platform to ensure the protection of juvenile and seagrass dependent fauna whilst enhancing marine biodiversity. Faunal response includes a broad aspect of behavioural characteristics including movement within the patch, species richness and presence and absence of the species. All species maintain different levels of sensitiveness to fragmentation and many studies have investigated the level at which species have been affected.
Keywords: Seagrass, fragmentation, habitat, biodiversity
Introduction
The monitoring of Common Eelgrass (Zostera marina) has a strong evolutionary history in UK coastal regions (Bell et al., 2011), yet there are several key areas of study that are poorly understood. Keywords ‘fragmentation, spatial, indicators and Zostera’ were used to identify peer reviewed journal articles which address the affects of Z.marina fragmentation on faunal abundance within the U.K. Z. marina meadows are globally recognised to be spatially dynamic due to natural threats, causing meadows to proceed in periodic movements in response to local variations of sediment erosion (Marsden & Chesworth, 2014) and it is reported that Z. marina is declining at an estimated rate of 7% per year (Macia & Robinson, 2005). Ecologists have noticed these reoccurring disturbances have evidently resulted in intensified habitat change, furthermore affecting the habitats ability to re-colonize and recuperate (Hughes et al., 2009). Further studies have investigated the affects of predation rates, especially on crab and bivalve species.
This literature review seeks to understand (a) the main drivers of habitat fragmentation (b) how the degree of fragmentation influences epifaunal density, diversity, and community composition and (c) the spatial and temporal scales used to describe organism response within fragmenting seagrass beds. In addition, understanding the variability and limitations of Z. marina growth patterns will enhance our ability to recognise ecosystems under stress and when management actions are needed most (Orth et al., 2006). Lastly, this review will highlight the significant gaps of knowledge within landscape ecology in order to improve our ability to conserve Z. marina around the U.K.
THREATS
Fragmentation does not occur as a single event, but is a developing, dynamic process and therefore it is most accurately assessed by long term research studies within the field. Growth dynamics of Z.marina as a result of multiple anthropogenic and natural stressors create complex arrangements over wide spatial scales. Therefore, it is important to identify the mechanisms that maintain and also disrupt patch dynamics in order to assess the response of associated fauna. Studies conducted in Porthdinllaen bay have assessed the impact of a local invasive macro-algae with the common name Japweed seaweed (Sargassum muticum). Due to the loss of Z. marina around moorings at this site, sedimentation has increased and the expanse of S. muticum is said to be increasing (Ref).
Z. Marina serve as the foundation of several community structures, each with fundamental and diverse ecological roles which provide foraging and protection opportunities for the growth and survival of juvenile fauna, including primary production, nutrient cycling, organic carbon production, sediment stabilization and trophic transfers to adjacent habitats. Z. marina also performs as an important resource for many Z. marina dependent species, some of which are economically important such as Hippocampus zosterae (dwarf seahorse) and Chelonia mydas (green sea turtle) (Orth et al., 2006). Previous studies reveal that the biggest cause of Z. marina habitat loss is due to various anthropogenic stresses, such as mining, trawling, and urban sprawling. Other concerns include intertidal vehicle use for unsustainable fishing practices, anchoring and pollution from industry, where such activities promote rapid fragmentation rates (McCloskey, 2013).
CURRENT UK STATUS
Dr Richard Unsworth of Swansea University said seagrasses were like the "canaries of the sea" in that their condition can be used as an indicator of the health of coastal waters. Table 1 represents the optimal measurements required by abiotic factors for maximum growth potential by Z.marina in the UK. A number of Z. marina studies have provided scientists including valuable information on water quality, affecting meadow health along the UK coastlines. Jones & Unsworth (2016) claim in the Royal Society Open Science journal that their study provides the first strong quantitative evidence that Z. marina of the British Isles is mostly in poor condition in comparison with global averages, with tissue nitrogen levels 75% higher than global values.
Their study included a long-term assessment of Z. marina among 11 different sites within the UK. Two of these studies focused on fauna response and five assessed the the spatial extent and overall quality of Z. marina. At each site, qualitative descriptive information was gathered about the perceived presence of anthropogenic impacts in order to place the data in context. All survey sites measured a depth between 0–3 m. The results show the Isles of Scilly to possess the highest nutrient balance and overall health. This would be predicted due to how remote the islands are and therefore less environmental impact causing physical damage (Figure 1).
METRICS AND ASSESSMENT TECHNIQUES
A study conducted in 2013 used a series of biochemical and morphometric parameters to assess the Z. marina in 14 meadows around the UK. This enabled information about light availability, nutrient status and general plant health to be determined.
Based on the assessment of quadrats along transects whilst scuba diving. This enabled detailed quantitative data to be collected within these quadrats (either as photos, shoot harvest or underwater observations). Most studies have used transects perpendicular to the shore, leading to the eventual depth maxima of the meadow. Such an approach is not always applicable when the meadow is very large and expands in different directions.
ASSESSING FAUNAL RESPONSE
Comparative studies of patchy and continuous habitat assessment are essential to understanding the consequences of habitat heterogeneity and fragmentation. Through the studies of Hughes et al (2009) and Robbins & Bell (2000), it has been proven that faunal species show various types of response to habitat fragmentation. Some are advantaged and increase in abundance, while others decline and become locally extinct. Understanding these diverse patterns and the processes underlying them is an essential foundation for Z.marina conservation (Borum et al., 2005). Quality of habitat, fragment shape and the extent to which the wider landscape isolates populations are attributes that influence the absence or presence of species. McCloskey (2013) demonstrates that for many marine taxa, abundance is positively correlated with fragment size, yet little evidence exists that identifies specific taxonomic groups to be sensitive to habitat edges. One study supports this calim as they found no correlations between Z. marina loss and species richness, total faunal density or diversity within their small sample areas. However, larger sample sizes with 90% habitat removal had significantly lower faunal species richness and total faunal density than plots with 0, 10 or 50% habitat removal, suggesting that beyond a threshold level of disturbance, species richness and abundance rapidly decline (Reed & Hovel, 2006).
In order to determine the sensitivity of a particular species respective to its habitat, one can record movement patterns of certain species periodically. Additionally, food availability is a critical factor that leaves a long term affect on species assemblage. A study investigating abundance by Macreadie et al. (2009) showed the dominating species to be Pipefish (Stigmatopora), which are highly specialized organisms and very dependent upon Z. marina throughout their lifecycle. The primary food source of Stigmatapora species is small planktonic crustaceans that are carried to Z. marina beds by water currents (Murphy et al., 2009).
AFFECTS OF LANDSCAPE CHANGE ON SPECIES ABUNDANCE
Z.marina research has recently been integrated into the study of landscape ecology, yet our understanding of faunal sensitivity to change in Z.marina habitats is still deficient. Most studies on fragmentation are undertaken where individual fragments are proposed as the main study. However, to reach a conclusion based on the consequences of landscape change, it is critical to compare landscapes as a whole (McGarigal & Cushman, 2002). Three interrelated processes take place when comparing whole landscapes. These include a reduction in the total amount of the original vegetation (i.e. habitat loss); subdivision of the remaining vegetation into fragments (i.e. habitat fragmentation); and lastly the introduction of new land to replace the previously missing vegetation. Moreover, different species have different ecological attributes, such as their scale of movement, life history stages, longevity, and what they may constitute as a habitat (Mazerolle & Villard, 1999). In turn, each factor effects how an organism may judge the suitability of a habitat as well as its ability to survive in a modified landscape. Consequently, different taxa may perceive a particular landscape differently with regards to habitat suitability.
Collins et al., (2010) stated that the size of a population is determined by the balance between four variables. These are births, deaths, immigration, and emigration. In fragmented habitats, these variables are influenced by several categories of processes. For example, the result of faunal isolation in fragmented habitats may affect various types of movement, e.g. seasonal or migratory. Ecological processes within fragments also experience ongoing changes in the years after isolation because of altered species interactions (Burdett & Watts, 2009). When a fragment is isolated, numbers of fauna does not immediately fall to a level proportional with its long-term carrying capacity. Instead, ‘species relaxation’ occurs, where numbers decline periodically, therefore it is difficult to measure at what stage the habitat is undergoing the full affects of fragmentation.
One study has directly assessed faunal response and spatial change between healthy and fragmented meadows by using artificial Z. marina habitats over one month. Conducted by Macreadie et al., (2009) they used four artificial treatments (control, fragmented, patchy, and disturbance control). They found up to 39% fewer species were present after 1 week in the patchy treatment, yet species richness in fragmented treatments remained similar to the control treatment. Overall, total fish abundance did not vary between all treatments and therefore the total fish count remained unaffected.
Similarly, a second study by Orth et al., (2006) used the approach of directly assessing the role of predation within Z. marina to assess spatial structure. Marine Biologist Elke Bojanowski (2017) supported this idea by claiming that the loss of keystone species e.g. Tiger sharks which predate in the shallow waters, have a lasting effect on the spatial and trophic structure of marine ecosystems. The absence of Tiger sharks would cause the meadow to become overgrazed and heavily fragmented due to an increased numbers of sea turtles, thus many other organisms would become absent. In a study by Reed & Hovel (2006) the rate of survival in juvenile blue crabs (Callinectes sapidus) rapidly declined where predation rates have dropped. Organisms have also been seen to spread to patch edges when predation rates are lower, thus patches become smaller as they continue to graze.
Conclusion
Current research has engaged in few effective conservation strategies of Z. marina ecosystems. McCloskey (2013) has proposed three key actions which are needed to ensure successful management. These include the development of a coherent worldwide monitoring network, the development of quantitative models predicting the responses of Z. marina to disturbance and finally the education of the public on the functions of Z. marina beds and the impacts of anthropogenic activity. Furthermore, Z. marina is persistently ignored during the zoning process for marine protected areas, especially within the tropics. Many fish species that inhabit coral reefs depend on adjacent Z. marina meadows which hold significant foraging and protection values, yet these are frequently ignored in policy and practice. Developing a greater understanding of the faunal linkages between Z. marina and associated coastal habitats can facilitate more informed ecosystem level management. Impediments to addressing the issues of Z. marina conservation include uncoordinated research by unskilled scientists and studies being limited to certain bio-geographic areas of the country. Furthermore, marine policies and regulations are consistently being violated or there is a lack of implementation of laws altogether.