Interesting(?) information re seagrasses

[Habib]

Below given abstracts might be of interest to some.

The first abstract tells that oxygen deprived substrate inhibits seagrass growth. However, in this respect, differences have been found between differences species.


Aquatic Botany
Volume 65, Issues 1-4, November 1999, Pages 175-197

Are seagrass growth and survival constrained by the reducing conditions of the sediment?

J. Terrados, , a, C. M. Duartea, L. Kamp-Nielsenc, N. S. R. Agawinb, E. Gaciab, D. Lacapd, M. D. Fortesd, J. Borumc, M. Lubanskic and T. Grevec

a Instituto Mediterráneo de Estudios Avanzados (CSIC-UIB) Edificio Mateu Orfila, Campus Universitario UIB Carretera de Valldemossa, Km, 7.5, 07071 Palma de Mallorca, Spain
b Centro de Estudios Avanzados de Blanes, CSIC, Camí de Santa Bárbara, s/n. 17300 Blanes (Girona), Spain
c Freshwater Biological Laboratory, University of Copenhagen, Helsingorsgade 51. DK 3400 Hillerød, Denmark
d Marine Science Institute, University of Philippines, Diliman, Quezon City 1101, Philippines



Abstract
A literature review of the effects of the reducing conditions of the sediment on seagrass metabolism, growth and survival, and of the morphological and physiological adaptations that seagrasses show to cope with sediment anoxia is presented and major gaps in knowledge are identified. The hypothesis that sediment anoxia controls the survival of seagrasses was tested experimentally by increasing the oxygen demand of the sediment with the addition of sucrose. Experiments were performed in a tropical (Southeast Asia) multispecific seagrass meadow, a Mediterranean Cymodocea nodosa meadow, and a temperate Zostera marina meadow. Sulfide levels in pore water and vertical redox profiles were used to characterise the effects of the sucrose additions on the sediment, while plant responses were quantified through the changes in shoot density and leaf growth. Sulfide levels in pore water increased and sediment redox potential decreased after the addition of sucrose to the sediment of different seagrass meadows. The effect of the addition of sucrose to the sediment of seagrasses was species-specific. Leaf growth was reduced and shoot mortality increased in some of the tropical species (e.g., Thalassia hemprichii), but not in others. Neither mortality nor leaf growth of the Mediterranean species C. nodosa was affected by sucrose additions, and only leaf growth was reduced two months after the addition of sucrose in Z. marina. Our results suggest that increased sediment anoxia might be a factor promoting growth inhibition and mortality in seagrasses, although strong differences have been found among different species and environments.

 

[Habib]

Here is one in which nutrients were added.

As expected nitrogen seems to be important.

Aquatic Botany
Volume 65, Issues 1-4, November 1999, Pages 123-139

Nutrient limitation of the tropical seagrass Enhalus acoroides (L.) Royle in Cape Bolinao, NW Philippines

Jorge Terrados, , a, Nona S. R. Agawinb, Carlos M. Duartea, Miguel D. Fortesc, Lars Kamp-Nielsend and Jens Borumd

a Instituto Mediterráneo de Estudios Avanzados (CSIC-UIB) Edificio Mateu Orfila, Campus Universitario UIB Carretera de Valldemossa, Km. 7.5, 07071 Palma de Mallorca, Spain
b Centro de Estudios Avanzados de Blanes, CSIC, Camí de Santa Bárbara s/n, 17300 Blanes, Girona, Spain
c Marine Science Institute, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines
d Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark



Abstract
Experimental additions of nutrients to the sediment of Enhalus acoroides stands were performed at four sites and three times along the year in Cape Bolinao, NW Philippines to test the hypothesis that seagrass growth in tropical environments is limited by the availability of nutrients. Both the nitrogen content (as % DW) and the nitrogen incorporation of E. acoroides leaves increased after the addition of nutrients. The size (g DW per shoot) and the leaf growth rates (g DW per shoot d-1) of E. acoroides shoots also increased after the addition of nutrients. Nitrogen rather than phosphorus was the nutrient limiting shoot size and leaf growth of E. acoroides in the area. The extent of nutrient limitation of E. acoroides showed high variability both in space and time which cannot be directly linked with differences in light or nutrient availability among the experimental sites

 

[Habib]

Here is one with IMO various interesting information.
A.o. the loss of nitrogen from the leaves. Seagrass might not be a good way to export ammonia and nitrate.


Aquatic Botany
Volume 65, Issues 1-4, November 1999, Pages 141-158

Leaf nutrient resorption, leaf lifespan and the retention of
nutrients in seagrass systems*1

M. A. Hemminga, , N. Marbà1 and J. Stapel

Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, P.O. Box 140, 4400 AC, Yerseke, The Netherlands



Abstract
Efficient nutrient resorption from senescing leaves, and extended leaf life spans are important strategies in order to conserve nutrients for plants in general. Despite the fact that seagrasses often grow in oligotrophic waters, these conservation strategies are not strongly developed in seagrasses.

Abstract
A compilation of literature data on nutrient resorption from seagrass leaves shows that the mean resorption of nitrogen is 20.4%, and that of phosphorus 21.9%, which is lower than comparable values for various groups of perennial terrestrial plants. The actual realised resorption in seagrasses may be even less as a result of premature losses of leaf fragments due to herbivory and hydrodynamic stresses, and due to leaching losses.

Abstract
The leaf lifespan in seagrasses on average is 88.4 days, but is highly variable, ranging from 345 days in Posidonia oceanica to only a few days in Halophila ovalis. Leaf lifespan increases with increasing leaf weight, and decreases with increasing leaf formation rate. Furthermore, leaf longevity increases going from tropical to temperate latitudes. We compared seagrass leaf lifespan with those of freshwater angiosperms, terrestrial herbaceous plants, shrubs and trees. Considerable variability in leaf lifespan was also found in these plant groups, but comparison among data sets shows that seagrass leaf lifespan is significantly lower than the leaf lifespan of terrestrial herbaceous plants, shrubs and trees. No significant difference was found between the leaf lifespan of seagrasses and freshwater angiosperms.

Abstract
Leaves are usually the major sink for nutrients in seagrasses. The combination of low nutrient resorption from the leaves and a short leaf lifespan is, therefore, expected to result in a low nutrient residence time in the plants. Indeed, field experiments with 15N labelled Thalassia hemprichii showed that less than 5% of the initial 15N amount was still within the living plant biomass 240 days after labelling.

Abstract
Limited nutrient retention in the plant biomass necessitates the capture of new nutrients for persistent growth. We speculate that effective nutrient uptake by seagrass leaves is an important strategy to maintain an adequate nutrient balance in seagrasses, particularly in thin vegetation or in small patches. The constraints imposed by the marine environment may have favoured the development of this strategy over the development of efficient nutrient conservation strategies

 

[K. Lee]

Here is one in which nutrients were added.

As expected nitrogen seems to be important.

Aquatic Botany
Volume 65, Issues 1-4, November 1999, Pages 123-139

Nutrient limitation of the tropical seagrass Enhalus acoroides (L.) Royle in Cape Bolinao, NW Philippines

Jorge Terrados, , a, Nona S. R. Agawinb, Carlos M. Duartea, Miguel D. Fortesc, Lars Kamp-Nielsend and Jens Borumd

a Instituto Mediterráneo de Estudios Avanzados (CSIC-UIB) Edificio Mateu Orfila, Campus Universitario UIB Carretera de Valldemossa, Km. 7.5, 07071 Palma de Mallorca, Spain
b Centro de Estudios Avanzados de Blanes, CSIC, Camí de Santa Bárbara s/n, 17300 Blanes, Girona, Spain
c Marine Science Institute, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines
d Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark



Abstract
Experimental additions of nutrients to the sediment of Enhalus acoroides stands were performed at four sites and three times along the year in Cape Bolinao, NW Philippines to test the hypothesis that seagrass growth in tropical environments is limited by the availability of nutrients. Both the nitrogen content (as % DW) and the nitrogen incorporation of E. acoroides leaves increased after the addition of nutrients. The size (g DW per shoot) and the leaf growth rates (g DW per shoot d-1) of E. acoroides shoots also increased after the addition of nutrients. Nitrogen rather than phosphorus was the nutrient limiting shoot size and leaf growth of E. acoroides in the area. The extent of nutrient limitation of E. acoroides showed high variability both in space and time which cannot be directly linked with differences in light or nutrient availability among the experimental sites

About the Nitrogen found as the limiter over Phosphorous in this study, it may be real fundamental, or even off the wall, but, could it be that of the two, the molecular bonds of the molecule containing Nitrogen yields more energy then breaking the bonds of the phospphorous molecule? I'm not going to try to extrapolate the math, because as I said this could be really basic, and very simple for a chemist to do. It would take me hours to come up with a proof on the otherhand, IF possible. Both Nitrogen, and Phosphate are rather inert by themselves, as I understand it, but the energy release from the bonds of molecules for the respective atoms must be easier with Nitrogen, or no? I find the study interesting as the two elements considered are both fairly inert, but apparently hold some, if not alot of sway in most things, life included.

Habib, why was the study done?

 

[billsreef]

Interesting reading, not sure what to make of it yet though

 

[greenighs]

I'm with you, Bill. I just planted a couple dozen seagrass plants (Thalassia among them) and now I'm wondering if I did the right thing.

I have a 65-gallon tank, for captive-bred seahorses (arriving soon), and I planted it to give the horses a home as near to their natural environment as I could. I have about 45 pounds of live rock, a 4-inch sand bed with grasses and C. prolifera and an 8-inch planter section with some of the grasses and some red kelp.

Since seahorses are slow feeders, I know I'll have a lot of uneaten frozen mysis sinking to the bottom. I was hoping it would serve as a nutrient source for the grasses and macros. Sure hope my tank plan doesn't backfire!

 

[Plantbrain]

Some thoughts:

The ammonium ion is much easier than the nitrate for the plant to assimilate, it's already reduced. But if the levels are too high in the water column, other opportunistic algae will bloom.

Critter/fish waste is the best choice for this for an aquarist.
NO3 can be dosed or low levels from critter/fish waste can also supply enough in many cases. Depends on the balance(eg a 120 gal reef and a 20 gal refugium). A 120 gal reef tank full of critters will supply enough N for most seagrasses.

While aquatic angiosperms assimilate nutrients from the water column, they also assimilate from the substrate when the water column nutrient supply is limiting. These are plants with true roots.

Grain size of the substrate is important as it relates to flow rates within the substrate. The variability in the research site's locations is somewhat expected and species to species. It is doubtful that the flow rates/grain sizes, nutrients within the soil substrates where similar. Pore water is a good indicator for nutrients within the root zones. It's a messy job but some poor folks have to do it(....like me)

But be careful here.
One species might not like it much, while another might show insignificant results or grow much better in the same location.

Another thing: the sites.
Are these sites the best optimal places for the plants? Yes, they exist there, but that does not mean they would not do better in another artifical environment. They grow there because that's where they _can_ grow, not because this is the best place for them. Be careful assuming that nature provides the best possible home for a species to thrive. Many species are quickly outcompeted by other species when given optimal conditions in nature and are regulated into these harsher environments as a result.

Aquarist need to be cautious when applying these studies to their tanks.

Generally all the seagrass beds I've ever swam through are quite anoxic and foul smelling below 2-10cm. I would expect the redox to decrease and the sulfides to increase whether there were plants there or not... if you added sucrose.

O2 supply to the roots:
Aerenchyma formation takes sometime.
Loading more carbon(sucrose) into an anoxic root zone would overload the needed supply of O2 on this anatomical feature. It would take a few months/weeks to adjust. Perhaps some of these issues were accounted for in the study. Maybe not.
But adding carbon to the substrate would place a O2 stress on the O2 rhizoshpere around the roots. Plants actively pump O2 down into their roots zones. Our DSB's are pretty aerobic vs many natural seagrass beds.

Shoot density/length does NOT indicate increased growth. This leaves out roots/tubers/stolon formation.
Relative growth rates/dry weights would be a better indicator of plant growth. One/other studies uses DW's. Also what about epiphytes on the leaves of the seagrasses? How do these effect the plant's growth/absorption of nutrients?

I have not read each of these research articles recently. I have read most of the Aquatic Botany journals the last 4 years, my old professor and dept chair are/were editors for it. It's a very good journal. I may thumb through them again but I did not recall anything that offered much useful insight in to the tank culture of seagrasses. But often that's all you have to start on.

I would really stress being careful as an aquarist applying ideas/suggestions from these research articles. They are often extremely specific to a given set of conditions or species of plant/algae/critter etc. If the suggestion is similar to your argument, and the application sound(Same species, the general priciples are close, other confounding issues N/A), then it's likely good to use it to support your argument.

Regards,
Tom Barr