Defining Marine Ecosystem Health

The concept and challenge of marine Ecosystem Health

The marine ecosystem is an important biological system that provides services to humanity as a whole and therefore we are a beneficiary of the ocean for what it provides, but research is stressing the urgency for environmental management of this ecosystem due to the rapidness of its declining state. \citep{Costanza1999} 
Primising to prevent and assess this declining state, is to understand the basic concept of marine ecosystem health. Underlying of Ecosystem health is the idea, that ecosystems have an optimal, healthy and natural status, which is urged to protect and retain. But what means healthy in this context and what kind of properties has a marine ecosystem to display to be classified as healthy ? Marine Ecosytem health is difficult to define, because as an open system, it is in a constant flux and exchange with its surrounding. Furthermore there are different perceptions of the elements which belong to a marine ecosystem. Generally it can be said that a marine ecoystems consists of the abiotic environment and the biotic community but its argued if this includes humans as a property of this constuct as well. Based on this, marine ecosystem health is not seen as a single feature, but as the sum of contributions of the biotic community, meaning individuals and populations and the abiotic processes within the system. \citep{Tett2013}  
A healthy ecosystem is characterized as one that is capable of maintaining its vigor (overall metabolism and energy flow) and organization (diversity of interactions between ecosystem components) over time, in the presence of a stress (resilience). Focusing on the ecosystem resilience, it has two main components, the length of time (RT) that a system needs to recover from stress and the magnitude of stress (Ms), from which the system can return to its former state. \citep{Costanza1999} These factors are important because ecosystems show little change under a stress until they reach a critical threshold, related to RT and Ms. After this point, they react in highly unpredictable (non-linear) ways and this may lead to their collapse. \citep{Scheffer2015}  
As it is beneficial to understand where an ecosystem was prior to stressors, there are common symptoms in which signal ecosystem degradation; they have been identified from reoccurring stressor impacts and described with the ecosystem distress syndrome concept.
Degradative symptoms characterise many ecosystems that are experiencing multiple stressors. \citet{Rapport_1992} claims that aquatic ecosystem distress syndrome can include some or most of the symptoms listed: alteration in biotic community structure to favor smaller forms, reduced species diversity, increased dominance by 'r' selected species, increased dominance by exotic species, shortened food-chain length, increased disease prevalence, and reduced population stability. \citep{Rapport_1992a}
To assess the dilemmas which are faced when understanding  ecosystem health the application of a human health metaphor is suggested. There are three fundamental issues which are proposed: recognizing the absence of objective criteria for assessing health indicators, recognizing when changes are actual or a false alarm, and the quest to find classifying indicators with attributes of being holistic, early warning or diagnostic. \citep{Rapport_1992a}
Above those issues, we face a two-fold challenge of boosting ecosystem behaviour and multiple stressor knowledge and working on the development of an identification system of indicators while we deal with the anthropogenic stressors which impact the integrity of biological ecosystems. \citep{Rapport_1992a}

Ecosystem Indicators

Scientists’ interest in choosing appropriate indicators for evaluating the ecosystem health has grown over the last two decades and nowadays ecological indicators are used widely to communicate information about ecosystems and the impact human activity has on them. Indicators are usually used to track negative change or degradation of an ecosystem. They often demonstrate damage that has already occurred and provide little information about future degradation. \citep{Hattam2015}
For better results and precise outcomes in the evaluation process, indicators must be carefully selected, in order to represent ­­­the various aspects of an ecosystem. Any indicator should be sensitive enough to provide an early warning of change, be able to detect changes over a wide geographical area and capable of providing a continuous assessment over many different stressors. Furthermore, an indicator should be relatively independent of sample size, easy and cost-effective to measure, collect and calculate and able to understand the differences between the natural cycles or trends and the human induced stressors. \citep{Rombouts_2013}

Biomarkers

Thermal stress is one indicator in the present height of research and concern. As anthropogenic activity increases the temperature of the ocean, many marine communities show signs of stress as an effect of the ocean absorbing carbon dioxide. Evaluating certain aspects of the ocean can help us understand, predict, and hopefully slow down the impacts imposed by human existence. But how can we start evaluation of stress? Researchers look for biomarkers which can indicate  environmental change. \citep{Sammarco2013}
Physical appearance is a possible key biomarker, a quantifiable characteristic in which can be measured and can help identify a biological process,  of stressor impacts on coral reefs as \citet{Cooper2009} point out a prime example of this in the symbiotic relationship between corals and microalgae, and the joint pigmentation they produce. Within coral live photosynthetic dinoflagellate, a type of microscopic algae. The two organisms live in symbiosis; the coral providing accommodation, carbon dioxide, and nutrients while the algae supply molecular oxygen produced by photosynthesis and also fixed carbon which aids in the coral’s energy requirements. \cite{Sammarco2013,Solayan2016,corals} Brightness in corals is a direct effect of the corals having light-absorbing compounds that result in pigmentation but they are aided by the pigments found in their symbiont photosynthetic microscopic organisms. \citep{Cooper2009} Measuring this pigmentation is a potential assessment tool in assessing water quality and furthermore, the overall health of the ecosystem. Water quality is defined as a change in its chemistry, including changes in the concentration of the amount of nutrients, rates of sedimentation and levels of turbidity’. Considering that the symbiotic pair is sensitive to water quality shifting, any changes will directly affect the microorganisms in the possible changes described before. Therefore, by measuring the pigmentation of both coral and the microscopic algae, one can gain an understanding of the surrounding water quality. \citep{Cooper2009}.
Another efficient tool for scientists who want to evaluate the health of an ecosystem is the use of empirical estimated indicators, which are categorized into primary producers and primary consumers.

Trophic level indicators

Organisms are organized in different trophic levels according to their feeding relationships. \citep{Trites2001} The energy in the ocean is transferred by photoautotrophic organisms, or primary producers, placed in the first trophic level to herbivores, or primary consumers, that form the second trophic level. They are followed by the carnivores, also called secondary and tertiary consumers, that occupy the third and last trophic level. In addition, there is a group called decomposers, existing on waste and dead remains, that breakdown all the organic matter. \citep{Sumich2016} All of them form a food web in which all the organisms rely on the organic matter produced by primary producers. In the ocean, the principal primary producers are phytoplankton, zooplankton are the primary consumers and the rest are secondary and tertiary consumers. \citep{Gaedke2009}
Trophic cascade is defined by Herendeen as the change in the stock of one trophic level when the stock of another trophic level is changed. \citep{Herendeen2004} They are the consequences of a higher trophic level on their following one. \citep{Pace2013} Even though this last case is the most common, infectious agents can also produce a trophic cascade. \cite{Buck2017}
Moreover, trophic interactions are defined as relationships between different organisms residing in the same area. These interactions are symbiotic if at least one of the involved species is benefited. They can be classified in three subgroups: mutualism-benefits to both, symbiont and host, parasitism-provides benefit to the symbiont to expense of the hostand in commensalism the symbiont benefits from the host without causing any effect on it. \cite{Sumich2016}
Trophic levels in marine ecosystem can be used as indicators to provide information of the health condition. Distinction has been made between single and multiple trophic levels indicators, as described in the following paragraph.

Single trophic level indicators

 Primary producers are indicators which consist of the phytoplankton and the phytobenthic groups. Tracking phytoplankton’s fast growth rates and response to low levels pollutants, scientists can identify sensitive and early signs­­ of ecological changes and environmental disturbance in marine surface waters. \citep{PAERL_2003} Phytobenthic groups are sensitive indicators of changes in marine environments as they live long and they usually are sessile organisms. \citep{Borja_2008,VanHoey2010} Macrobenthic communities also undergo chronic disturbances which affect their diversity, total abundance and total biomass and this may help the scientists identify modifications in the trophic structure and functioning of an ecosystem. \citep{Borja_2008,Dauvin_2007,Warwick_1994,article}
Primary consumer indicators include the zooplankton and zoobenthic groups. In 2005, the professor Weijerman mentioned that the abundance of zooplankton species can be used to document sudden ecosystem shifts in different regions of the world and also help identify the effects of global warming and its consequences on regional hydrodynamics. \citep{Beaugrand2009} As regard to zoobenthic groups, their structural characteristics can be useful to monitor the impacts of environmental stress in coastal systems \citep{Dauvin2007} and they can also be used in detecting early environmental disturbances, especially in estuarine habitats. \citep{Dauvin2007,Elliott2007}

Two or more trophic level indicators

The restriction of using these empirical estimated indicators, however, is that they can demonstrate the state of an ecosystem examining only a single trophic level. The need for assessing the ecosystem health in two or more trophic levels led scientists to introduce different approaches.
The use of stable isotope profiles as environmental and trophic indicators is a supporting tool to study feeding habits, nutrient tracking, species migrations and habitat use of species and communities in the ecosystem. \citep{Thompson} In addition, modelling techniques have been introduced to study ecosystem responses to different stressors. \citep{Christensen2007} An ecosystem model can be used to compute a group of indicators describing the physical attributes, lower trophic levels (phyto- and zooplankton mainly in pelagic systems), higher trophic levels, and ecosystem principles. Furthermore, an interesting advantage of using modelling techniques is that they can simulate future ecosystem states of the ecosystem under various scenarios, and predict the values that an indicator would take under different situations. \citep{Rombouts_2013}
Another approach, proposed by \citet{Hattam2015}, using the Dogger Bank as a case study, underlines the need to link indicators into three main classifications. Ecosystem services, the first classification,  is “the direct and indirect contributions of ecosystems to human well-being” (TEEB, 2010). Ecosystem functions, the second classification refers to ecological mechanisms which regulate the flow of energy, nutrients and organic matter \citep{Cardinale2012} . The classification ecosystem benefits describes the processed output from ecosystem services, which is therby no longer linked to the ecosystem  \citep{Haines-Young2012} . Identifying sufficient service, function and benefit indicators remains a challenge due to the complexity of marine ecosystems. 
However, once the proper indicators for each classification are chosen, and they are assessed in conjunction, they can give a more complete understanding of how an ecosystem change. \citep{Hattam2015}
This change leads us to the different impacts causing the ecosystem change and their short and long term considerations.

Short term and long term considerations

 As already shown before, using an already known concept to describe a new problem could help to simplify and understand several complex topics. So there was an application of a human health metaphor suggested to transfer this on assessing ecosystem health using different indicators. \citep{Rapport_1992}
Due to the fact, that the main research goal of this project is to contribute to greater understanding of how to assess the impacts and effects of multiple stressors in marine ecosystems in our current time of climate and environmental change, we should try to simplify this complex web of different stressors interacting with each other to get a general overview of the ongoing changes. Thus, it is beneficial to review the concept of assessing ecosystem health to understand the assessing of multiple stressors and their interactions.
The “aquatic ecosystem distress syndrome” lists different symptoms characterizing ecosystem health, such as: alteration in biotic community structure to favour smaller forms, reduced species diversity, increased dominance by ‘r’ selected species, increased dominance by exotic species, shortened food-chain length, increased disease prevalence, and reduced population stability. \citep{Rapport_1992} All these symptoms are abiotic and biotic factors in ecosystems and there are more, such as temperature, pH, nutrients, gas concentration, species concurrence, migration, etc. For all these abiotic and biotic factors one can apply the maxim for complex metabolic or ecosystem processes, called “Blackman’s principle”. According to this principle, the rate of the process is limited by the pace of the slowest abiotic or biotic factor, when the process is conditioned as to its rapidity by a number of separate factors. Blackman used this principle specifically to describe the metabolic process of photosynthesis. \citep{BLACKMAN_1905} To illustrate this, imagine a large wine cask, where one of the planks is shorter than the others. This means it is not possible to fill more wine in the cask than the high of the shortest plank. The shortest plank in this case functions as a metaphor for the limiting abiotic or biotic factor. 
Transferring this concept to an ecosystem with multiple interacting stressors, this could help to find a method for assessing multiple stressors and improve their impacts and effects on marine ecosystems. Considering different ways of classifying, grouping, quantifying and understanding stressors and the systems they impact at various organizational levels, according to the Blackman’s principle, there (probably) is a limiting factor.
In a study of the Australian Institute of Marine Science it is mentioned, that in a short time perspective, it could help to improve local stressors with an acute impact in marine ecosystems, but in long term there is most likely a stabilization of the ocean temperature needed to prevent the coral losses in marine ecosystems, because, in this case, temperature as a stressor causes a lot of other negative impacts and effects on the marine ecosystems. \citep{De_ath_2012} Thus, temperature would be the limiting factor or stressor. This means, even if one, theoretically, improves all other stressors, in a long term perspective not all changes in the ecosystem health could be prevented, due temperature rise would still have a quite big impact on the condition of the ecosystem.
The Blackman’s principle thus gives us a general idea on how to improve ecosystem health and minimize the impacts and effects of multiple stressors in a long term perspective by finding the limiting stressor, which is most likely the rise of temperature.
So all in all, the concept of ecosystem health could be useful to be able to assess and predict how marine organisms and their ecosystems will adapt and respond to different stressors acting in unison, and also which the most significant or substantial ones are, so we can suggest a systematic framework of these colossal changes to integrate the effects less universal stressors into.