Freshwater and marine ecosystems make up a huge portion of the earth’s surface. Within these ecosystems they support most of the biodiversity found on earth. In freshwater alone there are said to be 100,000 known species, Freshwater and marine diversity function as a valuable natural resource for economic, cultural, aesthetic, scientific, and educational terms. Throughout my literature review I will be providing an overview of the information gathered to compare freshwater and marine ecosystems. Functioning of the ecosystems will be a main point for the topic. Nutrient Cycling which provides us with evidence that the limiting factors within each ecosystem shape and determine the functioning within. A key factor to the production, and functioning of marine and freshwater ecosystems comes from the nutrient availability. limitation of phosphorus in freshwaters is demonstrated within many levels, this is not the case for nitrogen in marine ecosystems, therefore nitrogen limitations within this ecosystem remains an unanswered question. Species Biodiversity is the empirical evidence found in a study conducted by Covich (1999) suggested that changes in the biodiversity and species richness in the benthic zone have high variability in terms of effects on ecosystem functioning with direction of response. The Relative factors reviewed can be a strong selective force which shapes each of these ecosystems and communities.
Marine and freshwater ecosystems are diverse, complex environments. Learning how these aquatic ecosystems respond to their environment and certain stressors allows us to understand so much more about these ecosystems. Although this is a broad topic, different mechanisms can be considered to help us understand the differences between these two environments, and why they are occurring. By combining some of my own field research during my time spent in Russia studying freshwater and marine ecosystems, and studies conducted by scientists within this field I will make comparisons and contrasts between the two different environments. Key factors of these ecosystems are the functioning of these ecosystems and how they work. Nutrient limitations show us what type of environment this is, and species biodiversity or composition of the area all define the certain types of ecosystems. The basic purpose of this literature review is to provide a general over view from my findings from the literature I studied, pertaining to functioning of marine and freshwater ecosystems, and species composition and biodiversity found within each.
Functioning of Marine and Freshwater ecosystems
A key factor to the production, and functioning of marine and freshwater ecosystems comes from the nutrient availability. In freshwater and marine ecosystems fish excrete nitrogen and phosphorus. These two nutrients are main components for growth of phytoplankton and many aquatic plants. In marine ecosystems nitrogen is the limiting nutrient, as compared to freshwater ecosystems where the identified limiting nutrient is phosphorus for primary production. The reason a nutrient may become limited is because not only is there not enough of this nutrient, but there is more than enough of everything else in this ecosystem that an organism needs to allow for production and growth. The current rates of nutrient limitations in freshwater and marine ecosystems can be very different. Using Phytoplankton helps to give us a greater understanding into nutrient availability because phytoplankton can become limited by the amount or availability of nutrients in terms of nitrogen and Phosphorus. In a study conducted by Hecky (1988) we learn that marine phytoplankton tend to be nitrogen limited while in freshwater phytoplankton tend to be phosphorus limited. Reviewing experimental data indicates that if there is a limitation of phytoplankton growth then there is also a limitation of phosphorous in freshwater environments that can be showed at different levels from algal cultures to entire lakes, within the complexity of the systems. Although the limitation of phosphorus in freshwaters is demonstrated within many levels, this is not the case for nitrogen in marine ecosystems, therefore nitrogen limitations within this ecosystem remains an unanswered question. Looking at freshwater ecosystems an explanation proposed by Hecky (1988) is that nitrogen compounds are considered more mobile than Phosphorous compounds. This results in nitrogen being more able to flow through terrestrial ecosystems and accumulate in freshwater areas. Due to the abundance of N in freshwater, P is relatively more limiting in freshwater. Phosphorous compounds are not very mobile, they tend to bind to the compounds within their environments such as soil and aquatic sediments. The lack of mobility from phosphorous molecules leads to phosphorus molecules getting bound up on land or in terrestrial ecosystems, resulting in nitrogen becoming more limiting then phosphorus in aquatic ecosystems. When P does enter freshwater ecosystems it often is inaccessible or unusable to primary producers (Stockner 1999) . This generalization is dependent on other factors within the ecosystem. In Marine ecosystems, Microbial Activity is a fundamental component of limiting nutrients and nutrient availability. Again, looking at Phytoplankton, we can see that they are responsible for most of primary production in marine ecosystems. The Nutrients in the upper ocean can majorly limit the abundance of phytoplankton. Experimental data found in a study conducted by Moore (2013) revealed two broad subjects of phytoplankton nutrient limitations. On Surfaces of low latitude productivity is limited by nitrogen availability, where the supply on the subsurface level is limited by iron when the nutrient supply is enhanced. Marine ecosystems are much more complex when looking at nutrient limitations but the knowledge is crucial for predicting how anthropogenic contributions will affect oceanic nutrients.
Nutrient cycling in marine ecosystems can come in many forms. Bottom up control predominates marine ecosystems. Bottom up control refers to ecosystems in which the productivity and nutrient supply work together with the primary producers within the ecosystems, and together control the structure of the ecosystem. An example of this which was found in a study conducted by Cury (2015) looking at plankton populations and how they are controlled by variables such as nutrient availability. Within a marine ecosystem the regulation of major food web components come from either primary producer, such as phytoplankton, or the input of limited nutrients. With an abundance of nutrients, the biomass of the primary producers will increase along with fitness of the population. Terrestrial ecosystems are mainly dominated by plants, (also primary producers) but the ocean is only covered by less than 1% plant biomass so the small minute abundance of plankton is what all the oceans animals are fed by. (Cury 2015) this can limit the productivity of the ecosystem. Bottom up control is shown by the marine ecosystem being less favorable for the primary producers, such as phytoplankton, which then has a negative impact on the abundance of zooplankton, which then leads to a decrease in the abundance of small prey fish, which eventually leads to a decrease in the abundance of the larger predators. In Addition to Bottom Up control, there is also Top down control. Top-down control plays a role in dampening ecosystem-level fluctuations. Top down control is when a top predator controls the structure and the population dynamics within an ecosystem. When I think of Top down control keystone species is what comes to my mind. One example I use to help myself understand this control is the sea otter’s effects on the kelp forests. without the sea otters preying on urchins, they would eat too much of the kelp which would cause that ecosystem to collapse or become unbalanced. With a decreased abundance of the top predators, the population of the prey increases majorly. With the increase of prey species leads to a decreasing zooplankton population, this decrease then leads to a reduction of grazing pressure on phytoplankton so they become more abundant. within the marine ecosystems predation mortality of unfavorable conditions for the top predator is said to be the major source of mortality for marine species. With high levels of mortality due to top down control, a dampening in ecosystems levels will fluctuate. The next Subject is on a species Level. Although Trophic cascades seldom occur, they can alter the productivity in marine ecosystems. Trophic cascades will occur when predators in a food web suppress the abundance or change the behavior of their prey. This will then release the next trophic level from predation. Trophic cascades can alter the abundance biomass and productivity of a population, community or across a trophic level in a food web. A trophic cascade implies keystone species, and top down control which was discussed in the previous question. Removal of a keystone species leads to a major change in the food web and ecosystem dynamic. Using my example from earlier with the sea otter, if you add an apex predator such as an Orca whale, you will limit the sea otter abundance, which will increase the sea urchin biomass. The reason that Trophic cascades seldom occur is because an apex predator such as an Orca, will be in an area with high prey biomass. With a high abundance of sea otters, this still means the sea urchin biomass will remain low due to the predation from the remaining sea otter population, resulting in weak grazing of the kelp by the urchins. Wasp waist control is the most complex system in functioning ecosystems in my opinion. Although it is the most complex it is also the most probable in an upwelling system small pelagic fish have showed to exert a major control upon energy flows in an upwelling system. small pelagic fish populations constitute a mid-trophic level population which can exert both top down control on the mesozooplankton, and bottom up control on top or apex predators. An abundance of prey fish will depend on the environment which will control the abundance of most predators also control the primary production of plankton. A decrease in prey fish shows a negative impact on predator abundance, which leads to increase in zooplankton and a high grazing pressure for phytoplankton leading to a decreasing population. within an upwelling ecosystem there is an intermediate trophic level which is occupied by a very limited amount of species which are intensely exploited and have a huge variation of abundance. When assessing states and changes within marine ecosystems we look at regime shifts to analyze important patterns and potential impacts on the nutrient cycles Regime shifts can alter the nutrient cycles and they can potentially have a huge impact on, ecosystem productivity. Global patterns of abundance over decades show us the existence of many stable states of pelagic fish, synchronized fluctuations of the fish populations looking at different ecosystems shower us a strong correlation between fish populations and climatic connections. A change within one state is expected to have a major impact on the functioning or the ecosystem. small pelagic fish are the main food source for many top predators including large pelagic fish, demersal fish and even marine birds and mammals. The collapse of a prey species brought on by climate change can be associated with a huge mortality rate of the top predators which feed on the population.
Biodiversity and species richness
Functioning of ecosystems have many other factors that play a role. Looking at the roles on a phytoplankton level regarding nutrient cycling, all the way up to Apex predators helps us to understand the differences between species and biodiversity in freshwater and marine ecosystems (Smith 2004). Creating connections allows us to link together the pieces together on an ecological level. Different freshwater and marine ecosystems may show different responses to the same changes in Biodiversity. To compare biodiversity in marine and freshwater ecosystems I will use the role of benthic species in freshwater and marine ecosystems. The empirical evidence found in a study conducted by Covich (1999) suggested that changes in the biodiversity and species richness in the benthic zone have high variability in terms of effects on ecosystem functioning with direction of response. Multiple studies with Macroinvertebrates were conducted, and they found strong correlations between functioning ecosystems and species diversity. Five of the seven studies exploring species richness in freshwater ecosystems conducted by Covich (1999) suggested that when biodiversity affects were shown, they occurred at low levels of species richness which was found comparable to processes in terrestrial ecosystems. In a study conducted by Cardinale and Palmer (2002) they looked at filter- feeding species from streams, specifically Net- spinning caddisfly’s. The impacts of dissolves nutrients such as Nitrogen and Phosphorous greatly impacted the species composition. Freshwater Biodiversity Is a huge conservation concern. Freshwater makes up only 0.01% of the worlds water and only 0.8% of the earth’s surface (Sannadurgappa 2011 ).
Although freshwater ecosystems make up a tiny portion of the earth, it contains a huge portion of the worlds biodiversity, containing 100,000 species. This is about 6% of all earths species. Freshwater is an extremely valuable resource, facing many Anthropogenic consequences. Species Biodiversity on a Marine ecosystem level is also a Fragile matter. Changes in Biodiversity affect the rates of ecosystem processes in both environments, yet marine ecosystems represent the most extensive habitat on earth and has the highest rate of species loss. Looking at benthic ecosystems such as bottoms of rivers, wetlands, lakes, and oceans lets us understand more about species richness in these areas because they have a major importance of high biodiversity and global significance for nutrient cycling. To incorporate my research during my time conducting field research in Russia I will also discuss species in an arctic environment. The arctic contains an abundant and wide range of freshwater and marine ecosystems. I will be focusing on the Freshwater located around the Zvenigorod Biological station such as the Moscow river, And the Marine ecosystem found at the white sea biological station, the white sea. This broad, wide range of freshwater ecosystem types contain a variety of habitats each with different levels of ecological complexity. At Zvenigorod we studied the entomology of marine insects. One species that can help us to understand the functioning of this ecosystem would be Euphmeptera or commonly known as mayflies. This species has a distinct and primitive life cycle, which makes it vulnerable to changes in its environment. An appearance of Mayflies means that this is a nutrient rich environment. In a study conducted by McCrackin and Jones (1999) They measured the recovery of lake Erie’s levels of dissolved oxygen based on the return of mayflies in the area, the return of the mayflies showed that the nurtients were once again rapidly converted into a usable source to consumers rather than accumulating in the muds. Without species such as mayflies in the benthic zone, many nutrients would reach concentrations too high to be usable to consumer species. Species within the freshwater communities rely and depend on how benthic species contribute to this complex System. Species found within the benthic zone of the arctic also played a huge contributing role. The Ophiuroidea, or commonly known as the brittle star, is another species we found an abundance of in the benthic zone, but this time in the White sea. Echinoderms like the Brittle star are only found in marine environments, characterized by radial symmetry with a body form consisting of five snakelike arms. Echinoderms such as the brittle star Dominate the Benthic layer in marine environments, unlike freshwater. According to a study conducted by Ambrose (2001) they play a huge role in carbon cycling and remineralization especially in the arctic environments. Even though the arctic has constant low temperatures, benthic communities within these ecosystems still can remineralize carbon as efficiently and as fast as ecosystems in lower latitudes (Howarth 2003) this study suggested that carbon cycling occurs due to the role played by infauna and megafauna of this community. Species who inhabit these ecosystems play a vital role in the functioning of each specific ecosystem.