1. The effects of global climate change and other associated
anthropogenic influences on sea grass populationsand
productivity
Matthew G. Highnam
October 20th, 2014
Tropical Marine Ecology
EBIO3190-001

2. Global climate change is a term that has been very prevalent throughout the
scientific community in recent history. It refers to all of the climatic changes that have
been occurring on earth, with most of the changes being anthropogenic, or human-
induced. Marine scientists have focused on how these climatic changes could potentially
be affecting marine ecosystems such as coral reefs, mangroves, and salt marshes.
However, these endangered ecosystems have been highly publicized in scientific studies
and the general media, causing them to greatly overshadow an even more threatened and
ecologically significant ecosystem called sea grass meadows. Sea grass meadows are an
assemblage of coastal marine flowering plants that have adapted to a fully submersed life
in the world’s oceans through the evolution of unique ecological, physiological, and
morphological adaptations such as internal gas transport, epidermal chloroplasts,
submarine pollination, and marine dispersal (Neckles et al., 1999). Despite these
adaptations, sea grass meadows have experienced drastic declines in their global
populations and total area covered. Extreme management and conservation efforts for sea
grass meadows need to be prioritized globally because their declines are primarily due to
global climate change and other associated anthropogenic factors, they are essential to
preventing the collapse of many coastal marine ecosystems, and they are one of the most
productive ecosystems on earth.
The unique adaptations that sea grasses have developed over evolutionary time
have actually made them more susceptible than most ecosystems to global climate
change. This is mainly because of two aspects of their adapted life beneath ocean’s
surface, which are that they are light-limited in that they have a maximum depth and

3. minimum light requirement for photosynthesis, and they are easily outcompeted for food
and light resources by organisms such as phytoplankton and macro-algae (Kim et al.,
2007). However, normally these aspects would not be of much concern if sea grasses
were living under normal, gradually changing climatic conditions. Unfortunately for sea
grasses, many highly respected marine scientists believe that they are “facing a global
crisis due to a diverse array of pressures from human activities in the coastal zone”
(Carruthers et al., 2006). This global crisis is real as there are many anthropogenic factors
that directly influence sea grass declines such as dredging and filling, certain fishing
practices, human population growth, and widespread coastal developments being
constructed (Short and Wyllie-Echeverria, 2009). There are also direct natural impacts
such as storms and increased turbidity of water but these could also be viewed as indirect
anthropogenic impacts because increase in global temperatures has been shown to
increase storm frequency and intensity, which in turn increases turbidity, or wave
movement, under the water’s surface (Neckles and Short, 1999). Overall, there are two
environmental parameters that have been shown to impact sea grass loss in all coastal
ecosystems and these are decreased water quality and increased sea level. These are both
the direct result of the over-arching, human-induced impact of global climate change.
Rising sea levels create a substantial problem for sea grasses because all species are
adapted to have a maximum depth limit (Kim et al., 2007). If the sea level rises, then sea
grass species, especially on the edge of the meadow, will not be able to receive enough
sun light and thus not be able to perform photosynthesis. Decreased water quality occurs
directly because of the increasingly large human populations and increasingly widespread
occurrence of chemically involved agricultural practices. These two factors give way for

4. excess amounts of sediments and nutrients to enter the water column, which consequently
block sunlight from reaching the bottom and inhibit photosynthetic processes for sea
grasses (Kim et al., 2007). This then allows for populations of micro-algae and
phytoplankton to thrive and for eutrophication processes to begin occurring, which result
in massive reductions in sea grass biomass (Short and Wyllie-Echeverria, 2009. This type
of large-scale reduction in biomass, usually couple with one or multiple other minor
anthropogenic impacts described above, is what has typically caused most of the total
loss in biomass of sea grasses over the past century.
Sea grasses may be very susceptible to global climate change influences and other
human-induced activities, but they are absolutely essential in other coastal marine
environments for preventing the collapse of the ecosystem in the face of climate change.
Sea grasses are very efficient as stabilizing sediments and filtering out nutrients from the
water column, as previously mentioned. By doing this, they are greatly limiting the
possibility for phytoplankton and micro-algal species to flourish in the ecosystem. If sea
grasses did not do this or were absent from an ecosystem that started experiencing excess
sediment and nutrient runoff, then micro-algal and planktonic species would
exponentially increase in density leading to algal bloom formation and eutrophication
processes, ultimately leading to the collapse of the entire coastal ecosystem (Lloret et al.,
2008). This relationship can be observed through studying the coastal lagoon of Mar-
Menor in southeastern Spain. Most lagoons consist of benthic macrophytes, which are
either sea grasses or macro-algae (Neckles and Short, 1999). This particular lagoon’s
bottom is covered with a bed of macro-algal species called Caulerpa prolifera. It was

5. observed that there was a “high benthic macrophyte biomass, which contrasted with the
low planktonic density and the good water quality” (Lloret et al., 2008). This relationship
demonstrated in the study highlighted the existence of a benthic control of the system
since benthic primary production is much more important than planktonic production.
Over the past decades, this lagoon has become subject to one of the largest tourist areas
in its region as well as the use of intensive irrigation agricultural practices. These exterior
changes have had direct impacts on the composition and processes occurring in the
lagoon. The region in which the lagoon is in is semi-arid, highly saline, and has very
scarce precipitation as rain only occurs in periodical flooding events. Thus, because of
the poor and impermeable soil and the agricultural irrigation, the large amounts of
nutrients and sediments that flow down to the lagoon during episodic rainfall events is
greatly increased. One would expect to see a eutrophication process begin to occur in the
lagoon due to the fact that the irrigation cropping has saturated the groundwater table and
now there is a steady flow of water reaching the lagoon from the watershed most of the
time. However, this was not the case as what was seen contrasted reports from other
lagoons that underwent a similar shift in the flowing of nutrient and sediment amounts
into them. The lagoons in the other reports had high nutrient and subsequent
phytoplankton concentrations and consequently had very low benthic macrophyte
biomass and eutrophication processes were occurring. In this lagoon, there were still
relatively low nutrient and sediment concentrations and no reduction in benthic
macrophyte biomass or in water quality. It was found that due to the extremely large
cover of benthic macrophyte biomass on the bottom, which was not nearly as large in
other reports, eutrophication did not occur. This signifies that benthic macrophytes play

6. an extremely significant role in the interception of water-column nutrients and retention
of sediments. Overall, this leads to the conclusion that benthic macrophytes can greatly
enhance a coastal ecosystem’s resistance to eutrophication (Lloret et al., 2008). In
relation to sea grasses, this means that the loss of them and the subsequent impact on
coastal ecosystems can be extrapolated from the Mar-Menor lagoon case study to say that
many coastal ecosystems would collapse under the presence of excess nutrients and
sediments in the water column, allowing algal blooms and eutrophication to occur.
Sea grass meadows are quietly one of the most productive ecosystems on earth.
One of the main reasons for this is that they can be considered as biological sentinels or
“coastal canaries” (Calladine et al., 2009). This means that they are essentially ecosystem
engineers as they provide a vast number of valuable ecosystem services. One service they
provide is an enormous source of carbon to the detrital pool that actually exports some
off to the deep ocean, meaning that sea grasses provide a supply of organic matter in an
extremely food-limited environment (Duarte, 2002). Also in regards to carbon, sea grass
meadows have much of the excess organic carbon buried within their sediments, which
are then hotspots for carbon sequestration globally in the biosphere (Duarte, 2002). Sea
grass’ structural components also provide ecosystem services by modifying currents with
their leaves, rhizomes, and roots, as well as trapping and storing sediments and nutrients
and filtering nutrient inputs to the coastal waters (Duarte, 2002). Some of the other key
ecosystem services that sea grasses provide are nursery and habitats for ecologically and
economically important shellfish and finfish (Calladine et al., 2009) and their proximity
to other critical habitats, like salt marshes in temperate regions and coral reefs and

7. mangroves in sub-tropical and tropical regions, facilitates trophic transfers as well as
cross-habitat utilization by fishes and invertebrates (Calladine et al., 2009). All of those
services provided coupled with the rich biodiversity of species within meadows as to
compared to outside of them, makes sea grass beds extremely productive themselves as
well as in helping other ecosystems increase their productivity through transfer of energy
and resources. One of the key points to make as to why sea grass beds are more
productive than mangroves, coral reefs, and salt marshes is that they are distributed
universally around the globe, meaning they are a cosmopolitan species, in every region
except the highest of polar regions (Duarte, 2002). This is in comparison with mangroves
and coral reefs that are limited to a very small tropical region in comparison as well as
salt marshes that are limited to a very small temperate region in comparison.
A potential counterargument to the claim about the importance of sea grass
ecosystems over other coastal marine ecosystems and the effects of global climate change
and other anthropogenic factors on them would be that a rise in sea level could actually
favor sea grasses. Most sea grass species are adapted and do well in highly saline
environments, which would result from sea level rise and increased atmospheric carbon
dioxide concentrations dissolved in the ocean (Carruthers et al., 2006). People could
point back to how over the past millions of years there has been instances of all of these
occurrences such as reduced water quality, sea level rise, and large-scale loss of sea grass
biomass but they all ended up adapting and moving shoreward and then they showed an
increase in biomass.

8. The problem with that, which is represented graphically in the above figure (Calladine et
al., 2009) is that the rates and total change in percent that have been observed over the
past one-hundred years is nearly ten times greater than what any previous sea grass
species prior experienced. The change of these factors was very gradual for sea grass
ancestors, and that is not the case anymore, which mean that we do not know if sea grass
species will be able to adapt fast enough or move fast enough with the drastically

9. increased rate of change. Also, moving shoreward is increasingly become an unviable
option because of the extensive and widespread coastal developments by human
populations (Short and Wyllie-Echeverria, 2009).
If you look at all of the many human-induced factors and even the natural
disturbance impacts affecting sea grass meadows, it’s clear that most of the more
detrimental ones are driven by global climate change. Global climate change and the
every-increasing human population are driving sea grasses to rapid rates of loss in total
biomass and total area occupied. With their increased susceptibility, if the rate of change
and the human-induced activities are not kept in check, our most productive ecosystems
on the planet, our coastal marine ecosystems, will collapse and eutrophication will take
over filling once areas of massive diversity to areas of algal blooms. The most difficult
issue facing conservation efforts is implementing management plans to reduce nutrients
and sediments from both diffuse and point sources in surrounding watersheds of marine
coastal ecosystems (Kenworthy et al., 2006). However, the only thing to do is raise
awareness about the simple, but extremely productive and vital marine ecosystem known
as the sea grass meadow that has been dying underneath the shadows of much less
abundant, productive, and distributed ecosystems of salt marshes, mangroves, and coral
reefs.

10. LITERATURE CITED
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Orth, F.T. Short, M. Waycott and S.L. Williams. 2009. Accelerating loss of
seagrasses across the globe threatens coastal ecosystems. PNAS 106:12377-
12381.
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A.R. Hughes, G.A. Kendrick, W.J. Kenworthy, S. Olyarnik, R.J. Orth, F.T. Short,
M. Waycott and S.L. Williams. 2006. A global crisis for seagrass ecosystems.
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