Feature Article: Water flow is more important for corals than light. Part II: The science of corals and water flow
In the first part of the article series I introduced the mechanisms by which water flow affects the respiration and photosynthesis of corals. To recapitulate, I described how rates of photosynthesis and respiration are driven by diffusion rates which are dependent on concentration gradients. Since concentration gradients are greatly affected by water motion, the respiration and photosynthesis rates of corals is strongly correlated with the intensity of water movement. This second article in the series will review some of the published scientific research describing how water flow affects coral health and in the next article I will discuss my own research demonstrating how flow speed and lighting intensity go hand in hand.
The fields of flow related coral research are inter-disciplinary and they rarely fall into a discrete category. To simplify the discussion I will break up the discussion of the effect of water flow based on arbitrary categories which I have loosely arranged based on a temporal scale. The short term effects of water flow which are nearly immediate can be observed or measured within minutes or hours. These short term effects include rates of photosynthesis, respiration, calcification, polyp extension, colony expansion, particle capture and pulsing. The intermediate effects of water flow are usually observable within days or weeks and these include growth, calcification, regeneration, temperature and light stress (such as bleaching), and colony orientation. On the longest timeline of weeks, months and years the effects of water flow are mostly discussed in terms of morphology, reproduction, dispersal, recruitment and ecology. Any one of these topics could fill volumes but I will focus my discussion on the topics which are most relevant to aquarium applications. The topics of focus will include photosynthesis and respiration, particle capture, bleaching, growth rate and the interaction between flow and morphology. There are innumerable publications available for these topics so I will select the studies which most clearly illustrate the importance of water flow to coral health from an aquarist’s point of view.
Short Term Effects
Photosynthesis and Respiration
The most fundamental of biological processes are photosynthesis and respiration. The response of photosynthesis and respiration to environmental factors such as flow is easily monitored by measuring oxygen or carbon dioxide consumption or production. Additionally, the photosynthetic efficiency can be evaluated by measuring electron transport rate within the photosystems.
In a beautifully simple experiment, researchers William Dennison and David Barnes maintained samples of Acropora formosa under stirred and unstirred lab conditions (1). The experiment showed that corals maintained under stirred conditions show significantly higher rates of photosynthesis, respiration and calcification than corals maintained in unstirred conditions. Corals maintained under unstirred conditions photosynthesized and respired about 25% less than corals in stirred conditions. The calcification rates of corals in unstirred conditions were also lower than corals in stirred conditions but this reduction was not statistically significant. The researchers concluded that the coral’s metabolic response to water motion would influence rates of coral growth and the development of reefs overall.
In 2003, Sebens et al. published a paper on the effects of flow on the physiology of Agaricia tenuifolia in Belize (2). Interestingly, the study showed that although the respiration seemed to increase proportionately with increased water flow, water flow speed had little effect on the maximum photosynthesis rate of A. tenuifolia. It appears that this species is able to carry out maximal photosynthesis even under greatly reduced flow which is in contrast to other coral species which have increased photosynthesis when they are exposed to higher water flow speeds.
One of the most thorough and detailed papers I have ever read about flow effects on coral physiology is “In Situ measurements of flow effects on primary production and dark respiration in reef corals” by Patterson et al (3). The researchers used sealed recirculating chambers deployed from the underwater research habitat Aquarius in Key Largo FL. They measured respiration and photosynthesis rates of Montastrea annularis under flow speeds of 2-16cm/s. Like Denison and Barnes the study also found that there was an increase of primary production and respiration with increased flow rates but more importantly, the discussion gives detailed explanations of the interactions between hydrodynamic forces and diffusional properties of gases to explain their results. The discussion goes far beyond the mechanisms I introduced in my previous article so if you are interested in learning more about how flow affects gas exchange in corals I would strongly recommend reading this paper.
When it seems that the majority of coral physiology research is performed on stony corals, the Dendronepthea research projects of Dr. Katharina Fabricius are available to help balance things out. In a classic study of Dendronepthea feeding behavior (4), Dr. Fabricius demonstrated the flow dependent particle capture of this beautiful coral group. When exposed to flow speeds from 0-35cm/s, over 200 samples from 35 colonies of D. hemprichi showed a significantly higher particle capture at the intermediate flow speed of about 15cm/s. At the lowest flow speeds, particle capture suffers from unfavorable hydrodynamics and a lack of particle “traffic” and at the highest flow speed, the polyps cannot remain open under the force of the water flow. Although the 1997 Sebens et al. (5) paper also deals with the effects of water flow on particle capture, it focuses on how the morphology of Madracis mirabilis interacts with water flow at different speeds to affect particle capture. Like the Fabricius paper mentioned above, the experiment determined that the highest rate of particle capture for M. mirabilis occurred at flow speeds between 10-15cm/s. However, whereas Dendronepthea feeding was largely shut down above flow speeds of 30cm/s, at flow speeds of 40-50cm/s M. mirabilis still showed about one fourth the overall capture efficiency demonstrated at the more favorable 10-15cm/s. The reason for this discrepancy is that whereas Dendronepthea polyps were largely collapsed under the force of the water flow, the solid branches of Madracis protected downstream polyps which remained open to capture food particles in the gentler turbulent eddies. Additionally, the Sebens et al paper showed that aggregations of Madracis are suited to capture particles under specific flow speeds depending on the density of the branches.
Medium Term Effect
Coral bleaching occurs when reef-building corals expel their life-giving symbiotic algae which makes them appear stark white. Although coral bleaching is mostly attributed to unusually high water temperatures, other factors such as turbidity and water motion seem to be decisive factors in making bleaching events more or less severe.
One of the most important coral conservation efforts is the science of determining where coral are most resistant to environmental stresses which cause coral bleaching and where they will be most likely to recover from those stresses. In a review of the published accounts of coral bleaching events, the paper by West and Salm (6) describes the relationship between coral bleaching and several environmental factors including water flow. Although large-scale weather patterns generate conditions which lead to mass bleaching, on a local scale the bleaching response is not uniform. One of the recurring details West and Salm discovered in their review is that corals which incurred little to no bleaching or rapid recovery almost always occurred in areas which were exposed to permanent or seasonal upwelling or which experienced rapid flow from being in channels or strong currents. The review concluded that conservation areas which were conducive to coral resistance and resilience to bleaching stress would likely be areas which experienced stronger and more consistent water flow.
Photoinhibition is a precursor to coral bleaching which occurs when corals receive more light than they can handle. When photoinhibition occurs, corals begin to stress and their photosynthesis rate decreases. In corroboration with the observations of West and Salm, the recent work by Nakamura et al. (7) has shown that there is a reduction of photoinhibition by water flow. By exposing samples of Acropora difitifera to varying water flow speeds and two different light regimes, Nakamura and his colleagues determined not only that photoinhibition is reduced at higher flow speeds but they also showed that at high light intensities and low flow speeds, photodamage of the coral symbionts is amplified which in turn makes corals more sensitive to light and temperature extremes. In a follow up to this experiment, the same group of researchers looked at the recovery rate of bleached Stylophora pistillata under similar experimental conditions (8). Since coral bleaching induces a loss of zooxanthellae, the researchers exposed the sample corals to low (3cm/s) and moderate (20cm/s) water flow speeds and measured zooxanthellae density within the corals over a 7 week period. While the corals from the moderate flow treatment showed a rapid increase in zooxanthellae density throughout the study, the low flow treatment corals barely gained any zooxanthellae density at all. Basically, the two aforementioned studies by Nakamura et al. support the position of West and Salm that water flow is important for making corals resistant to bleaching stressors and that adequate water motion is also important for helping corals recover from bleaching stress.
Although strong water flow is beneficial for helping corals resist bleaching, living in a really stable, comfortable environment can reduce a coral’s tolerance to occurrences of thermal stress. In the 2005 study by McLanahan et al (9), researchers surveyed reef zones around the island of Mauritius which differed mostly in their abundance of water flow. The study discovered that coral bleaching was most prevalent on the reefs which received the most constant and abundant water flow. Although this finding contradicts the majority of the literature regarding the relationship between water flow and coral bleaching, the paper explains that the most affected corals had reduced acclimation to minor stress events. Since the most affected corals were acclimated to a very stable temperature and constant high water flow environment, the corals did not have to acclimate to minor stress events so they were less able to tolerate an anomalous high temperature event. The ecologically dominant corals which suffered the most bleaching due to this temperature anomaly were Acropora and Montipora species.
The effect of water flow on growth rate is a multi pronged affair. Growth rate overall is a reflection of how well corals can photosynthesize, catch food, resist bleaching, avoid disease and predation. Growth rates are enhanced by flow due to decreased sedimentation, increased photosynthetic and respiration rates, greater abundance of food “traffic” and more efficient food particle capture.
Although this transplantation experiment by Dr. Ilsa Kuffner (10) seemed more concerned about the effects of ultraviolet light on the photoprotective pigments of corals, one of the factors which was varied was water motion. Dr. Kuffner transplanted replicate branches of nine different colonies of Porites compressa. The fragments were distributed between an ambient (normal) water motion area and a low water motion area. Corals transplanted to low flow showed decreased concentrations of photopretective pigments compared to the samples which were maintained in the normal flow environment. Additionally, the study determined that the only significant factor affecting calcification rate was water motion. Corals transplanted to areas of high water motion showed significantly higher rates of calcification than those which were part of the low water motion treatment. The aforementioned study by Dr. Fabricius (8) also showed than on a longer time scale the growth rates of D. hemprichi colonies reflect the efficiency of particle capture with growth rates being highest at flow speeds between 10-20cm/s. The Sebens et al (2) paper which studied Agaricia also performed measurements of Agaricia growth rates under different flow speeds. As expected, corals which grew in high flow environments also showed higher growth rates than corals which grew in lower flow environments; those which grew in sheltered concavities showed greatly reduced growth.
Long Term Effects
On a long time scale of weeks, months and years, the effects of water flow still include growth and calcification but the discussion of growth and calcification shifts away from measurements of length, area, volume and mass and we can begin to observe and discuss morphology, asexual reproduction, competition and survival.
Flow effects on Morphology
Corals are very adaptable animals and different morphs of the same species were often thought to be new species. Because corals live in a range of habitats, their skeletons have to grow into shapes and textures which will best suit the physical environment which is primarily governed by water flow. An older study by Dr. David Bottjer describes some interesting characteristics of Acropora cervicornis morphologies under high and low water motion environments (11). For this research project, Dr. Bottjer selected a low flow back-reef area and an adjacent high flow reef-crest area to investigate the branching density, angle and orientation of A. cervicornis colonies which grow in these different flow environments. Colonies growing in the calmer back reef area exhibited relatively upright branches with a great deal of spacing between the branches and no significant orientation with water flow. By contrast, the branches of colonies occurring in the high flow reef crest area were markedly different; the branches were much more densely spaced, they tended to be more horizontally inclined and the branches were mostly pointing away from the oncoming flow. The author concluded that the branch characteristics of high water flow colonies were well suited to reducing the transfer of momentum from flowing water to branches to reduce breakage. However, it is unclear whether this morphology is the result of a genetic response to water motion or the result of strong flow pruning branches and colonies which grew into ill suited shapes for the environment or both.
Although any aquarist or scientist will concede to the fact that water flow has a profound effect on the morphology of corals and other benthic organisms, very few explanations of the mechanisms which govern this flow-dependent property have been proposed. One exception is the 1985 paper by Rinkevich and Loya (12) on the branching pattern of S. pistillata. The paper cites previous work by the authors that showed that S. pistillata grows by dichotomous fission of the branches, that mature colonies are spherical in shape overall and that broken branches regrow faster than unbroken branches such that the spherical shape of the colony is mostly reformed (13). The authors were primarily interested in the curious fact that branches of S. pistillata never fuse even though branches which were broken and grafted to the same colony would rapidly fuse (14) (Note: go see for yourself, in your tank, if any of your Stylophora or Pocillopora branches have fused. Its likely they haven’t whereas Montipora and Acropora branches fused quite regularly in mature colonies). The authors proposed that an isomone (type of hormone) is responsible for preventing the fusion of branches of S. pistillata. To test this hypothesis the researchers attached upward growing branches of S. pistillata and Millepora alcicornis in pairs from the same colonies such that they would touch if they grew in a straight direction. Although the Millepora samples simply fused, the Stylophora samples grew very close to each other but then they simultaneously began to grow away from each other. If there is indeed a hormone which proportionately governs branch spacing, then water motion would have a profound impact on coral morphology; In low water flow the substance would be in higher concentration leading to widely spaced branches and in high flow the isomone would be less concentrated which would cause the coral colony to grow more closely spaced branches. Although it is quite possible that there are one or more hormones which control branch patterns in Stylophora, other Pocilloporids and other corals, I believe that polyp extension is a more plausible spacing control for this group. Since polyp extension can be largely explained by water flow as well, the physiological-repellent effect might be a factor of polyps touching each other when branches grow closely to one another, thereby stimulating the branches to grow away from each other.
Here is one for the zoanthid lovers. Although little consideration is usually paid to the flow requirements of “Zoa’s” as a group, a study by Dr. M. Koehl (15) produced a thorough investigation of the relationship between water flow and the morphology of zoanthid colonies. The species used were Palythoa caribaeorum and P. variabilis. P. carribaeorum colonies are characterized by having solid mad of interconnected polyps whereas the polyps of P. variabilis are about twice as tall and wide and they are only loosely connected. These two carribean species of zoanthids are usually found in environments with different amounts of water motion. It appears that the smaller polyps and more rigid colonies of P. carribaeorum are best suited to resisting drag forces which occur in higher water flow. As water flow becomes reduced, corals like zoanthids which grow close to the substrate start to experience difficulties with catching food, gas exchange and sedimentation. P. variabilis has developed several adaptations to overcome these difficulties. First of all, the taller polyps of P. variabilis stick out from the substrate into more mainstream water flow so that it experiences relatively similar flow speeds as P. carribaeorum even though it generally occurs in calmer environments. The longer tentacles and larger oral disc of P. variabilis allows it to catch more food particles even in reduced water flow. Also, the flexibility of the loosely connected polyps is useful for dealing with the increased sedimentation which occurs at lower flow speeds. Furthermore, P. variabilis lives up to its name because it has polyps which can look quite different depending on whether they are solitary or colonial. Since solitary polyps have no neighbours with which it can share the force of water motion, these tend to be shorter and thickened at their bases whereas colonial polyps are much taller and narrower at the base. On the contrary, polyps which occur at the leading edge of colonies tend to resemble solitary polyps since these also experience more force from water motion than polyps at the interior of zoanthid colonies.
Morphology effects on water flow
Just as water flow affects morphology, the morphology of corals is specially designed to alter water flow patterns to suit particular niches. In reality, water flow and morphology effects go hand in hand but it is clear that water flow affects morphology first. For example, the microhabitat which occurs at the interior of zoanthid colonies might foster increased growth rates and this phenomenon might help to explain why under certain conditions, small clusters of zoanthid polyps tend to multiply much faster than individual or loosely aggregated zoanthid colonies. But before zoanthids can create a suitable microhabitat for themselves, the solitary founder polyp must first acclimate to the water flow to which it is exposed.
As you might expect, a higher density aggregation of corals with many more upstream branches to disrupt water flow is more efficient at reducing high water flow than a lower density aggregation of the same size. Accordingly, Sebens et al (9) determined that denser aggregations of M. mirabilis were better suited to capturing food particles at high flow speeds because of their increased ability to effectively reduce water flow. However, the lower density aggregations with more spacing between branches were better suited to capture food particles under lower flow conditions because they allowed more water to pass through the colony. Although most of us think of morphology as the three dimensional shape of our corals (is. Massive, branching, plating), it also includes the 3d elements which project from the two dimensional surface to alter the texture or roughness of our corals. In 2001, Gardella and Edmunds (16) examined how surface roughness of two massive carribean corals affects the small scale properties of water flow and how this relates to diffusion and gas exchange. The experiment used differently sized skeletons of Dichocoenia stokesii and Stephanocoenia michilini and measured downstream turbulence under several flow rates. They compared the downstream turbulence to the surface roughness of their sample skeletons and they found a significant correlation in the smaller colonies they used. They found that the surface roughness between these corals showed little effect on turbulence although the taller Dichocoenia colonies had greater downstream turbulence, especially at higher flow rates. Therefore, the taller Dichocoenia colonies had a greater effect on turbulence which increased the potential for high diffusion and gas exchange rates.
I would like to reiterate that the effect of water flow on coral health is a continuum across the arbitrary topics which I have outlined. For example, the long term effect of high photosynthesis and calcification rates on a coral will yield a high growth rate and the long term effect of high growth rate will yield more sexual and asexual reproduction and it will also have an effect on morphology. Likewise, long term changes to coral morphology are usually to improve the characteristics of water motion through a coral colony to increase gas exchange and particle capture. In other words, corals depend on strong water flow for their immediate health and they grow into forms which reflect the flow conditions of their environment. To apply this to aquariums, you should not only consider how you can provide adequate water flow for your corals now but you should also be mindful of ways to keep up the flow characteristics of your tank as the corals mature and begin to alter and obstruct flow in your tank. In next month's article I will discuss some research of my own wherein I examined the effect of water flow at different lighting intensities on the photosynthesis and respiration of Pocillopora damicornis.
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