Experiment 2 using Fish Waste

Objectives

The objective for Experiment 2 was to investigate the potential use of fish solids high in nitrogen and phosphorous (both critical requirements for plant growth, maintenance, and carbon gain) removed in the initial treatment via the geotextile bags as a nutrient source for the nursery production of salt marsh plants to be used in coastal restoration. Plant biomass and nutrient allocation in Juncus roemerianus and Spartina alterniflora fertilized with a commercial plant fertilizer and solids from spotted seatrout aquaculture production was compared to determine the effect of these nutrient sources on plant productivity.

Methods

Plants of Spartina alterniflora and Juncus roemerianus were fertilized with (1) dried fish solids collected from the geotextile bags and (2) a commercial fertilizer (Miracle-Gro™). Plant morphology, biomass, and tissue nutrient content of plants dosed with fish solids and Miracle-Gro™ were each compared to the control independently due to different dosing concentrations to evaluate the potential of fish solids as a fertilizer source for coastal salt marsh plant species.  Both S. alterniflora and J. roemerianus were grown from native seed at the Coastal Plant Restoration Nursery located at the University of Southern Mississippi’s Gulf Coast Research Laboratory, planted in a 50:50 sand:topsoil mixture in 4-inch pots, and maintained in greenhouse culture. 

Fish solids were collected from the primary treatment portion (geotextile bag) of a small scale phytoremediation system designed for spotted sea trout (Cynoscion nebulosus) at the University of Southern Mississippi’sThad Cochran Marine Aquaculture.  At maximum stocking densities and feed rates for a small broodstock facility, approximately 3,500 liters of effluent at 4g/L was generated and pumped into a geotextile bag 3.0 x 3.7 m.  The geotextile bag was loaded up to 85% of its volume and allowed to drain and consolidate.  Dried fish solids were collected a few months after last backwash of the water filtration system. Before being applied to plants, fish solids were ground to consistency in a commercial blender, rinsed three times with deionized water, and dried at 65˚C. 

The nutrient addition experiment was conducted at the Thad Cochran Marine Aquaculture Center from September 2012 to March 2013.  Five pots of S. alterniflora and J. roemerianus were placed in shallow nursery trays for a total of 24 trays per species (5 pots x 24 trays = 120 pots/species).  Trays were randomly placed in a 3.0 x 3.7 m greenhouse and sub-irrigated daily.  For each species, eight trays were randomly assigned one of the following treatments (8 trays x 3 treatments = 24 trays/species): (1) Control (C) treatment, with the addition of 30-ml water, (2) Miracle-Gro™ (MG) treatment, with the addition of 0.87-g Miracle-Gro™ 20-20-20 dissolved in 30-ml water, and (3) Fish Solids (FS) treatment, with the addition 14-g of ground dried fish solids and 30-mL water (Fig. 2).  Miracle-Gro™ had 174 mg/dose of total nitrogen (present as 51.3 mg ammonium (NH4+), 52.2 mg nitrate (NO3-), and 70.5 mg organic nitrogen) and 174 mg/dose of total phosphorous (phosphate (PO43-)).  Dried fish solids had a total nitrogen content of 476 mg/dose (all as organic nitrogen) and 70 mg/dose of total phosphorous.  Dosing rates differed between fish solids and Miracle-Gro™ in attempt to be generous with nitrogen availability in the FS treatment due to the presence of nitrogen in fish solids as organic nitrogen only, which is less readily available for plant uptake. Nutrient additions were made weekly from September 21 to October 26, 2012, for a total of six weeks (dosing period), and plants subsequently remained in the greenhouse until March 18, 2013 (~20 weeks) to allow for conversion of assimilated nutrients into plant biomass during spring growth (response period).

Results

There was a fertilization effect of both nutrient additions (i.e., Miracle-Gro™ and fish waste solids) on S. alterniflora and J. roemerianus.  There was a weak growth response of S. alterniflora in the FS treatment, indicating that fish solids may not the most suitable nutrient source for this species.  However, there was a significant fertilization effect of fish solids on J. roemerianus, suggesting that these solids have potential as an alternative fertilizer in the nursery production J. roemerianus.  The growth response of S. alterniflora and J. roemerianus to fish solids found in the current study are similar to those reported in a previous study using shrimp biofloc solids. 

For each metric reported, the mean observation per flat (n=8) was calculated for each of the three treatments. These data were then transformed to Z-scores (obs-mean/stdev) for each metric to standardize the range of values (i.e., normalizing), and then analyzed using non-parametric multivariate tests in Primer v. 6.1.5 (PRIMER-ELtd) to visualize among treatment differences. First, the resemblance matrix (i.e., distance or similarity matrix) was calculated using Euclidean distance, where highly similar samples are grouped together. Second, the resemblance pattern in multivariate space was visualized using non-metric multidimensional scaling (MDS) with 50 runs, where points that are close together represent samples that are very similar. Third, the resemblance matrix was analyzed using analysis of similarity (ANOSIM) with 999 permutations to test the null hypothesis of no significant difference among treatments, followed by pairwise (posthoc) tests based on the difference of average rank dissimilarities to determine like groups.

Results obtained in the MDS plots provide a visual representation of the multivariate responses measured in S. alterniflora and J. roemerianus to the two nutrient enrichment treatments and the control at two time points, at the end of the 6 week dosing period and the end of the 20 week response period (Fig. 3). Stress for all four MDS plots was 0.2 or less, which is good for this type of analysis.  Results obtained by ANOSIM hypothesis testing indicate that there were significant treatment effects for S. alterniflora after dosing only (p<0.028), and for J. roemerianus at both time points (p<0.007 dosing and p<0.001 response). The pairwise comparison posthoc tests results indicate that whole plant responses to MG and FS were significantly different to each other, but not to the control in S. alterniflora at the end of the 6 week dosing period. In contrast, for J. roemerianus whole plant responses to the two nutrient enrichments were significantly different from the control, but not to each other at the end of the 6 week dosing period. Finally at the end of the subsequent the response period, J. roemerianus plants in the FS treatment were found to be significantly different from the C and MG treatment.

Figure 3: Multidimensional Scaling (MDS) plots summarizing the multivariate responses by two plant species to three treatments (Control = C, Miracle Grow = MG, Fish Solids =FS) at two different time points. (A) S. alterniflora at end of 6 week dosing period, (B) S. alterniflora at end of 20 week response period, (C) J. roemerianus at end of dosing period, (D) J. roemerianus at end of response period.

Conclusions

The results of the present study suggest the potential repurposing of reclaimed fish solids as a fertilizer for the nursery production of salt marsh plants for use in coastal restoration.  There was a relatively weak growth response of S. alterniflora to nutrient addition in the FS treatment, mostly related to positive responses in phosphorous uptake and incorporation.  However, the significant growth response of J. roemerianus plants in the FS treatment indicate the potential use of recovered fish solids from marine phytoremediation systems as a fertilizer for nursery production of this species. Over the past several decades, there have been increased efforts to restore salt marsh habitats degraded or destroyed from anthropogenic development, eutrophication and pollution, and extreme storm events (e.g., hurricanes) in order to reinstate lost ecosystem services, especially those related to carbon storage and fisheries maintenance.  These restoration projects often focus on replanting critical native salt marsh plant species that facilitate vertical marsh growth resulting in an increased demand for large-scale nursery production of native coastal salt marsh plants for restoration purposes. This increased demand could be met in part by phytoremediation, thereby offsetting known environmental impacts attributed to the marine aquaculture industry.