The fate of carbon in seagrass-dominated ecosystems

Fate of Carbon in a Sea Grass Dominated Ecosystem

Perhaps the most pressing concern for the world is the rising rate of global warming in the 21st century. Many discussions have taken place on the global front to discuss the possible steps to decrease this rate. On aspect that has been discussed diligently in relation to global warming is the rise of Co2. In a relevant research study, Longuhurst asserts that in order "to reduce the rate of global warming due to rising CO2, the potential for sequestering carbon by oceanic phytoplankton has received considerable international attention, which has culminated with a research agenda" (Longhurst 1991). Other researchers have also confirmed that the restriction of carbon under sea level as well as using the terrestrial structures to reduce Co2 rates are interesting topics due to the huge potential they have if used appropriately (Sampson 1992). The diagram below illustrate the levels of Co2 emissions in the ecosystem over a period of 2 centuries (Houghton et al., 1990)

This paper will aim to firstly review the carbon state in relation to global warming, land use and coastal structures as they stand today and secondly will analyze the data presented to present the fate of carbon in the global ecosystems.

There have been studies done in the past that have focused on the utilization of different aspects like kelps and photosynthetic responses in order to reduce carbon ratios in the global environments, but there is still very little evidence or focus given to the near-shore coastal structures and how they can serve in reducing the overall levels of carbon in the atmosphere (Titus and Stone, 1982). "The coastal system needs to be added to carbon cycle models because this sector (particularly in the tropics) has a high rate of carbon sequestration that has not been accounted for in terrestrial and oceanic carbon models" (Thom et al., 2001). The researchers add that the carbon reduction strategies can very effectively use the principles used in coastal ecosystem restoration to help countries reduce the carbon emissions (Thom et al., 2001).

Thom and colleagues in their study (2001) explain that "oceans play a key role in the global carbon cycle. Because of high primary productivity rates, relatively high nutrient concentrations, and coverage of the earth surface, coastal margins are an important component of the oceanic carbon cycle" (Thom et al., 2001). This is true as the overall productivity that takes place on the coastal margins is one of the primary energy sources for the oceanic life (Gacia et al., 2002). The fisheries on the coast also get a lot of their energy source from the coastal margins. With the increased importance of these coastal margins, there is a dire need for clearer understanding of the budget allocations needed. The relevant companies need to be completely aware of The various sources of carbon in the atmosphere,

The potential natural sources that help in its reduction as well as

The fate of carbon in the future

Clear definitions of these three aspects will help in the proper allocation of budget to help reduce global warming. Numerous international conferences were held to discuss the topic of necessary budgets in reducing global warming and in a summary of one important conference titled "Natural Sinks of CO2" in 1992, the researchers Wisniewski and Lugo (1992) asserted that it is extremely essential to incorporate "the coastal system along with the terrestrial, oceanic, and atmospheric systems in models of the carbon cycle because this sector of the biosphere (particularly in the tropics) has a high rate of carbon sequestration that has not been accounted for in terrestrial and oceanic carbon models." Including these will help in understanding how the importance of the coastal structure can assist in reducing the overall costs of global warming and carbon sinks as well as prevent it from resurfacing (Suzuki and Kawahata, 2003).

Carbon Sources

The primary sources of carbon in the near-shore coastal structures include the following:

Phytoplankton,

Benthic microalgae,

Seaweeds and Seagrass,

Kelp,

Tidal (fresh, brackish, salt),

Marshes and mangroves

The diagram above illustrates the numerous sources of carbon in the ecosystem, both natural and man-made.

Other important sources of carbon in the coastal areas include the dissemination of CO2 (dispersed), the expiration of marine producers (particles), and terrestrial and estuarine remains (these are in both the dispersed and particles form) (Valiela, 1984). "The relative contribution of terrestrially derived carbon (C/N >10) and marine-derived carbon (C/N

The table below illustrates the levels of carbon that exist in the world as of the statistics available in 2001 (Falkowski et al. 2001):

There are of course other incidences when carbon emission can be increased especially during the upwelling episodes where the light particles get re-suspended and hence emit carbon the environment (Valiela 1992). One researcher asserts that "changing CO2, temperature, wind and rainfall patterns and other factors would influence the rate and pattern of forest processes and succession" (Agren et al. 1991). In another study, the researcher explains that "increased temperature is expected to result in forest areas being replaced by grasslands & #8230; [hence] carbon processing and storage and nutrient dynamics will also be affected" (Anderson 1991).

An important aspect to consider in the fate of carbon in the ecosystem is the input of the territorial resources. The land uses of resources are critical in influencing the overall transference of energy and nutrients to the coastal structures (e.g. Correll et al. 1992). Kehoe in an earlier study (1982) asserts that "large amounts of carbon enter estuarine and coastal systems from watersheds, and it is clear that the rates and mass of nutrients, organic matter and sediments reaching estuaries will be altered. Logging, road construction, river channelization and development in watersheds have resulted in considerable increases in suspended sediments in streams and rivers. In turn, estuarine sedimentation and turbidity are the result of sediment from logging operations" (Kehoe 1982 as cited in Thom et al. 2001). The chart below illustrates similar findings (FAO Corporate Documentary, 2001):

For example, consider the diagram below which illustrates the association of the nutrient in the land and the carbon budgets allotted for the ecosystems in the forests. A Symbols are used in the diagram to indicate whatever changes or switches that take place in the budget allotted and how that is affected by the nutrients in the land or the plants (Melillo and Gosz, 1983):

When specifically focusing on the Grays Harbor estuary, Washington, USA, the overall ratio of sediment inputs is nearly five times more after the roads have been built nearby or the soil has been waterlogged. This was the case in the watershed located in the Chehalis River. The reason behind this was that the heightened level of turbidity influenced the structure so that it restricted the dispersion of sea grass in that particular coastal region (Suzuki et al., 2003). This is perhaps one aspect that needs to be studied more thoroughly to understand the impact of the processes of sedimentation and turbidity on the estuarine structures that can occur because of the anthropogenic activities that taka place nearby or in the same vicinity (Houghton and Woodwell 1983, Smith and Hollibaugh 1993).

Metabolism

The primary formula to calculate the metabolic structures of the coastal systems is (Ziegler and Benner 1998):

GCP = NPP + AR + HR

Where,

GCP = gross community productivity

NPP = autotrophic net primary productivity

AR = respiration by autotrophs

HR = respiration by heterotrophs (Ziegler and Benner 1998)

Relevant definitions include:

CR = community respiration = AR + HR

GPP = gross primary productivity = NPP + AR (Ziegler and Benner 1998)

These definitions are integral when understating how the metabolism of the carbon emissions in the ecosystem work, especially when calculating the irregularities on the carbon rates. Despite the usefulness of this formula, very few studies have used it to calculate the various sources of carbon and its potential sinks which leaves gaps in the knowledge of irregularities of carbon emissions (Tanaka and Nakaoka, 2007; Touchette and Burkholder, 2000). The following diagram illustrates the current cycle of carbon emission in the coastal and land ecosystems (FAO Corporate Documentary, 2001):

The diagram below illustrates the further division of the different land surfaces based on their ratio of carbon stocks and potential carbon emissions (FAO Corporate Documentary, 2001):

Carbon Fixation

The coastal structures like the benthic aquatic vegetation, upwelling areas amongst other have a much higher production and storage percentage of carbon then the biomass structures. This is why the overall turnover and transference of these smaller vegetations is also critical to consider when understanding the fate of carbon in the global ecosystems (e.g., Thom 1990). "Annual rates for tidal freshwater marshes, salt marshes, mangroves and seagrasses range from about 300-1000 gC m-2 & #8230; Algae, including seaweeds and kelps have NPP rates on the order of 400-1900 gC m-2 y-1" (Mann 1982).

The fact is that numerous rooted macrophyte structures are not full of naturally strong and healthy particles and sediments and nutrients. It is because of the restriction or absence of these particles, sediments and nutrients that the study of these systems has not been as extensive and thorough as the concentration on the terrestrial structures when understanding the fate, sources and sinks of Co2 levels in the ecosystems and the plants structures (e.g., Drake and Leadley 1991). Researchers assert that "rooted macrophyte systems can be sources of CO2, Chapter 4 and other gases through microbial processing of organic matter in the sediments and direct emission from leaves" (Delaune et al. 1990).

Table 1. Total net primary production (NPP) from world systems (Modified from Valiela, 1984)

Area

NPP

Tot. NPP1

% of Total

% of Total

106 km2

gC m-2 y-1

X106mTC y-1

System

Global

Marine System:

Open Ocean

46

15,355

74.1

24.1

Upwellings

0.4

74

0.4

0.1

Continental shelf

27

2,997

14.5

4.7

Algal Beds & reef

0.6

2.7

0.9

Estuaries (exc. marsh)

1.4

3.7

1.2

Tot. Marine

57

20,726

32.5

Continental System:

Terrestr. Env.

39,540

91.7

61.9

Swamp and Marsh

2

1,110

2,220

5.1

3.5

Lakes and Streams

2

0.7

0.5

Tot. Continental

43,112

67.5

Total Global

63,838

Total Benthic (Aquatic Vegetation)

3,552 5.6

(i.e., algal beds, reefs, seagrasses, mangroves, swamps and marshes)

1 This is (NPP in gC m-2 y-1) x (area).

Fate of Fixed Carbon

The reason that the carbon cycle is so important to the overall global ecosystem is because change in the carbon structure can result in the overall marine structure of our planer. The researchers Borges and Gypens (2010) in their study write that the "accumulation of anthropogenic CO2 in the ocean has altered carbonate chemistry in surface waters since preindustrial times and is expected to continue to do so in the coming centuries. Changes in carbonate chemistry can modify the rates and fates of marine primary production and calcification. These modifications can in turn lead to feedback on increasing atmospheric CO2." They further write that the alterations and switches taking place in the nutrient levels and sources of the rivers can elevate the irregularity of carbon chemistry in the atmosphere. They assert that these alterations mainly occur due to the management regulation policies that have been implemented over the years. This is one aspect that influences the nutrient levels on the coastal areas more so than oceanic acidification as well irrespective of whether the influence is positive or negative (Borges and Gypens, 2010).

This is why the fate of carbon in the ecosystems is an important topic of discussion in today's era. Especially when talking the near-shore coastal structures, the carbon chemistry and structure may be affected by the loss of carbon to the coastal sediments through the regular burial or recycling processes in the systems or though the consumption of herbivores or mere transference to the shoreline to dissolve with other organic substances. All of these aspects if not controlled or managed properly can result in irregular carbon chemistry and changes that could in turn impact the nutrient levels on the coasts (Schwarz et al., 2000; Raven and Falkowski, 1999; Plus et al., 2001; Philippart, 1995a and b; Ralph et al., 2007).

The assessments of the rates of carbon burial near the coasts are again very irregular and range from the low levels of 0.2 to as high as 1 cm y-1. This ratio is common for burial in most marsh structures though. The reason for this is usually the soil and seabed near the shores is already rich in nutrients with more than 25% of the accreting substances being completely natural and organic, with the accumulation ratios of more than ca. 4 gC y-1 (Thom 1992). "Carbon sinks as peat accumulation are great in some systems on the East and Gulf coastal where marshes have been forming for 3,000-4,000 yrs" (Bricker-Urso et al. 1989). In another study the researcher explains that "other marsh systems are very new, and have little surface peat accumulation. For example, salt marshes in the Pacific Northwest are buried on the average of once every 300 yrs by ocean sediments as a result of large earthquakes and land subsidence" (Atwater 1987). Even though this subject is not analyzed in most researches, this entire process can very efficiently restructure the entire march structure near the coasts and requisition the carbon percentages in the marsh forever.

"The estimates for direct consumption through herbivore pathways and detrital pathways are not well-known in general for these systems. What is evident is that large proportions of the standing crop of these plants can be removed by animals, which shred material and/or eat the material directly" (Adam 1990).

"The loss of 59.5 km2 y-1 in the Mississippi River deltaic plain due to reduced sediment input is an extreme example of what can happen" (Britsch and Kemp 1990). An important fact to note here is higher levels and percentages of floods can initially elevate the carbon fixation levels. The overall metabolic structures and procedures can also be changed with higher flooding instances. Considering this facet, Morris and his colleagues (1990) in their respective study analyzed and exhibited a very positive and strong association between the average sea level and the aggregate yearly production of carbon. They align this higher production levels with the increased levels of floods. When specifically analyzing the areas that have experienced higher floods, like the Mississippi delta, the assessments show that there is an elevated level of benthic respiration as well as a simultaneous decrease in the aggregate production levels. There is also a heightened level of and greenhouse gas emissions in the Mississippi delta due to the higher levels of flooding (Delaune et al. 1990).

In an earlier study conducted by Andrews and Abel (1879), the researchers focused on the use of photosynthetic pathway by the seagrasses to balance and manipulated the carbon levels on the coasts. This was of interest to the researchers as findings had indicated that the ratios of the sea grass carbon isotope was similar to that present on the C4 plants as oppose to the C3 plants which had been the previous consensus. The results of the study showed that the plant called T. testudinum could very easily be categorized as a C4 plant even though its leaf structure was very different from all the other known C3 and C4 plants in the territory. Furthermore, the plant had no indications of being similar or even near the Kranz structure as that was the structure most common for C4 plants to have. The study also established that there were many freshwater submerged aquatic plants had metabolic structures that resembled the C3 structures and had characteristics similar to many seagrasses taxonomically (Andrews and Abel, 1879; see for similar results Grice et al., 1996; Hellblom and Bjork, 1999).

"Changes in the stable carbon isotope composition of Thalassia testudinum (turtle grass) leaves were measured in response to in sltu light-reduction treatments in Tampa Bay, Florida, USA," in the recent study by Durako and Hall (1992). The results of the study showed that the "leaf SI3C values of shaded T. testudinum were significantly lower than those of unshaded controls in both shallow (0.75 m below MLW) and deep (2 m below MLW) sites. Changes in leaf 6I3C were correlated with differences in the relative amount of light reaching the experimental treatments, and the magnitude of the responses increased between 1 and 3 mo after initiation of the shade treatments. Because of the close proximity of the experimental and control sites, the decrease in 6I3C in response to shading probably reflects a process (i.e. isotopic fractionation) effect rather than a source (i.e. dissolved inorganic carbon) effect." In the study they concluded that during times when the light is significantly decreased so much so that it restricts the photosynthetic rates within the T. testudinum, the carbon chemistry and production increases significantly so much so that it appears to have no limits (Durako and Hall, 1992).

In an earlier study, Abel (1984) focused on the photosynthetic carbon increase of the Thalassia hemprichii (Ehrenb) which is a very common tropical sea grass). The researcher chose to study the phenomenon of using various methodologies. The results showed that the "photosynthesis in buffered seawater in media in the range of pH 6 to pH 9 showed an exponentially increasing rate with decreasing pH, thus indicating that free CO2 was a photosynthetic substrate. However, these experiments were unable to determine whether photosynthesis at alkaline pH also contained some component due to HCO3? uptake" (Abel, 1984).

In another relevant and recent study, Beer (1997) highlighted the level and extent to which another sea grass named Zostera marina L. was capable of making use of or exploiting the compound - HCO3. The study analyzed the use of the compound as the exterior inorganic-carbon resource to perform photosynthesis. The study also analyzed which possible combinations the compound was most useful in. The results showed that "HCO3? was used as a major source of inorganic carbon at the normal seawater-pH of 8.2, and that bulk CO2 contributed only marginally (less than 20%) to photosynthesis at the pH. By comparing photosynthetic rates at pH 8.2 and 9.0, it was deduced that CO32? could not be utilised. It was further found that HCO3? could be acquired via extracellular dehydration to CO2, as catalyzed by external/surface-bound carbonic anhydrase, prior to inorganic-carbon uptake" (Beer, 1997).

"In December 1999 heavy continuous rains that lasted one week affected the Venezuelan coastline. At Morrocoy National Park, a large marine reserve, rainfall values surpassed previous 32-year records and led to a decrease of salinity to 3 psu, which lasted for over a month at some locations. This study examined effects of these changes on the shallow-water meadows of the sea grass Thalassia testudinum Banks ex-Koning (1805), by comparing their structure before and after this disturbance at four selected sites" (Chollett, 2007). The impact that the rain had, according to the study, was that it played the role of a pulse-type turmoil in the ecosystem and changed the overall structure and facets of the physicochemical framework in the region. Even though the change occurred because of the heavy rain, the physiochemical structures returned to their original self pretty soon.