Brief description
Essential nutrients for seagrass growth may be derived from benthic decomposition of organic matter. Two experiments run consecutively from December 2003 to February 2004 tested this idea. Cores of Halophila ovalis (seagrass-vegetated) and unvegetated sediment (control) from the Swan River, WA were amended with either particulate organic matter (POM) or dissolved organic matter (DOM) to test whether a positive feed-back loop exists, where increased organic matter results in increased seagrass nutrients.Lineage
Maintenance and Update Frequency: notPlanned
Statement: - Plant collection -
The effect of addition of organic matter was investigated in two experiments, run consecutively in summer (December 2003 - February 2004). Cores (90 mm diameter PVC pipe) of Halophila ovalis with associated sediment were collected from shallow water (0.5 m deep) at Pelican Point in the Swan River (31 59 S, 115 49 E), Western Australia. Unvegetated cores were collected as controls from the same site, approximately 5 m from the nearest seagrass bed. Cores were collected with approximately 9 cm depth of attached sediment and retained in PVC tube. Initial leaf density within the cores was 40 +- 2.5 leaves per core. Biomass allocation was assumed to be constant across cores initially and this assumption
was supported by results from extra cores (not used in experiment) which showed that leaves accounted for 37 +- 1% of total biomass (n = 9).
The effect of addition of organic matter was investigated in two experiments, run consecutively in summer (December 2003 - February 2004). Cores (90 mm diameter PVC pipe) of Halophila ovalis with associated sediment were collected from shallow water (0.5 m deep) at Pelican Point in the Swan River (31 59 S, 115 49 E), Western Australia. Unvegetated cores were collected as controls from the same site, approximately 5 m from the nearest seagrass bed. Cores were collected with approximately 9 cm depth of attached sediment and retained in PVC tube. Initial leaf density within the cores was 40 +- 2.5 leaves per core. Biomass allocation was assumed to be constant across cores initially and this assumption
was supported by results from extra cores (not used in experiment) which showed that leaves accounted for 37 +- 1% of total biomass (n = 9).
Statement: - Experimental design -
Cores were placed into individually aerated aquaria
(dimensions of 10 cm x 10 cm x 25 cm) filled with river water. Cores were then left for two days to stabilize prior to organic matter addition. Throughout the experiment, water was refreshed with Swan River water twice weekly (volume exchanged ~0.75 L) and, as such, was not a closed system. A sub-sample of water-column water, collected when water was exchanged, was used to estimate potential nutrient export via this pathway. A minimum of 4 cm of water overlaying the sediment/water-column interface was maintained during water exchange so as not to expose the sediment surface to air. Cores were incubated under fluorescent lights (12 h light:12 h dark) providing approximately 300 mmoles of photons m-2 s-1 photosynthetically active radiaiton (PAR), which is above the saturating irradiance for growth of Halophila ovalis (Hillman et al., 1995). A total of 64 cores (2 organic matter addition types (POM and DOM), two sample types (vegetated or unvegetated) x 4 treatments x 4 replicates) were maintained at 18 degrees C for approximately four weeks. Experiments with the two types of organic matter were run consecutively with cores for DOM experiment collected approximately 4 weeks after the POM cores.
Concentrations of nitrogen (N) and phosphorus (P) in H. ovalis plant tissue are usually depleted over the growing season (Hillman et al., 1995), however, whilst there was significant depletion of leaf N (two sample t-test, P < 0.001), there was no significant change in starting leaf P concentrations (P = 0.368) between the two experiments.
Cores were placed into individually aerated aquaria
(dimensions of 10 cm x 10 cm x 25 cm) filled with river water. Cores were then left for two days to stabilize prior to organic matter addition. Throughout the experiment, water was refreshed with Swan River water twice weekly (volume exchanged ~0.75 L) and, as such, was not a closed system. A sub-sample of water-column water, collected when water was exchanged, was used to estimate potential nutrient export via this pathway. A minimum of 4 cm of water overlaying the sediment/water-column interface was maintained during water exchange so as not to expose the sediment surface to air. Cores were incubated under fluorescent lights (12 h light:12 h dark) providing approximately 300 mmoles of photons m-2 s-1 photosynthetically active radiaiton (PAR), which is above the saturating irradiance for growth of Halophila ovalis (Hillman et al., 1995). A total of 64 cores (2 organic matter addition types (POM and DOM), two sample types (vegetated or unvegetated) x 4 treatments x 4 replicates) were maintained at 18 degrees C for approximately four weeks. Experiments with the two types of organic matter were run consecutively with cores for DOM experiment collected approximately 4 weeks after the POM cores.
Concentrations of nitrogen (N) and phosphorus (P) in H. ovalis plant tissue are usually depleted over the growing season (Hillman et al., 1995), however, whilst there was significant depletion of leaf N (two sample t-test, P < 0.001), there was no significant change in starting leaf P concentrations (P = 0.368) between the two experiments.
Statement: - Organic matter addition -
Organic matter was added to the cores in two forms, particulate organic matter (POM) containing 58% C by weight, and dissolved organic matter (DOM) containing 42% C by weight. POM, in the form of wrack, was collected from Woodman Point, Western Australia (32 08 S, 115 44 E) and consisted primarily of leaves from the seagrass Posidonia. Wrack was dried, ground to pass though a 1 mm2 sieve (to standardize POM addition) then soaked in river water and added as a layer (without covering the leaf blades) on the sediment/water-column interface. Treatments were 0, 1, 5, 12 g POM dry weight per core. Each gram of POM corresponded to a nutrient enrichment of 602 mmoles N and 21 mmoles P (C:N:P ratio of 1994:29:1) if POM mineralized completely. Prior to enrichment, the sediment contained ~1% dry weight of carbon, so a 12 g addition of POM would correspond to a 3-4 fold increase in organic matter
in the top 5 cm of sediment, if organic matter was considered to be homogeneously mixed into the upper layer.
DOM was added into the root zone (to simulate root released DOC) by inserting diffusion tubes containing sucrose 5 cm below the sediment surface. The average length of Halophila ovalis roots was ~5 cm (unpublished data), although roots could be as long as 12 cm. There were three treatments (0.8 g, 2.4 g, and 5.2 g) and a control (an empty tube). The 0.8 g treatment was attained with 1 small tube (8 cm long sucrose-filled reticulation pipe e 6 mm wide, plugged at each end with seven evenly spaced 1.5 mm diameter holes). The 2.4 g treatment was attained with 3 of the small tubes described above. The 5.2 g treatment was attained with 4 large diffusion tubes (8 cm long sucrose-filled tube e 8mm wide, plugged at each end, with seven evenly spaced 2 mm diameter holes)
For further methodology see Chapter 2 of the thesis.
Organic matter was added to the cores in two forms, particulate organic matter (POM) containing 58% C by weight, and dissolved organic matter (DOM) containing 42% C by weight. POM, in the form of wrack, was collected from Woodman Point, Western Australia (32 08 S, 115 44 E) and consisted primarily of leaves from the seagrass Posidonia. Wrack was dried, ground to pass though a 1 mm2 sieve (to standardize POM addition) then soaked in river water and added as a layer (without covering the leaf blades) on the sediment/water-column interface. Treatments were 0, 1, 5, 12 g POM dry weight per core. Each gram of POM corresponded to a nutrient enrichment of 602 mmoles N and 21 mmoles P (C:N:P ratio of 1994:29:1) if POM mineralized completely. Prior to enrichment, the sediment contained ~1% dry weight of carbon, so a 12 g addition of POM would correspond to a 3-4 fold increase in organic matter
in the top 5 cm of sediment, if organic matter was considered to be homogeneously mixed into the upper layer.
DOM was added into the root zone (to simulate root released DOC) by inserting diffusion tubes containing sucrose 5 cm below the sediment surface. The average length of Halophila ovalis roots was ~5 cm (unpublished data), although roots could be as long as 12 cm. There were three treatments (0.8 g, 2.4 g, and 5.2 g) and a control (an empty tube). The 0.8 g treatment was attained with 1 small tube (8 cm long sucrose-filled reticulation pipe e 6 mm wide, plugged at each end with seven evenly spaced 1.5 mm diameter holes). The 2.4 g treatment was attained with 3 of the small tubes described above. The 5.2 g treatment was attained with 4 large diffusion tubes (8 cm long sucrose-filled tube e 8mm wide, plugged at each end, with seven evenly spaced 2 mm diameter holes)
For further methodology see Chapter 2 of the thesis.
Notes
CreditStrategic Research Fund for the Marine Environment (SRFME)
Issued: 04 12 2006
Data time period: 2003-12 to 2004-02
text: westlimit=115.5; southlimit=-32; eastlimit=116.5; northlimit=-31.5
Subjects
63 605002 |
Biogeochemical Cycles |
EARTH SCIENCE |
Halophila ovalis |
OCEAN CHEMISTRY |
OCEANS |
Oceans | Marine Biology | Marine Plants |
dissolved organic matter |
nitrogen |
oceans |
particulate organic matter |
phosphorus |
sediments |
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Other Information
(PhD thesis)
uri :
http://theses.library.uwa.edu.au/adt-WU2007.0016/
global : 71787310-59c9-11dc-9ffa-00188b4c0af8
Identifiers
- global : 58040920-59e3-11dc-9ffa-00188b4c0af8
