Experiment 1 using Shrimp Waste

Objectives

The objective for Experiment 1 was to estimate the impact of nitrogen and phosphorous inputs from two anthropogenic sources (i.e., commercial fertilizer and aquaculture solid waste) on Gulf Coast salt marsh plant productivity.  Plant biomass and nutrient allocation in Juncus roemerianus and Spartina alterniflora fertilized with a commercial plant fertilizer and solid biofloc waste from shrimp 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 a commercial fertilizer to represent excessive nutrient inputs from agriculture run-off and dried shrimp biofloc solids to represent aquaculture-related nutrient inputs.  Plant morphology and biomass, tissue nutrient content, and leaf chlorophyll content were compared among treatments (i.e., control, fertilizer, and shrimp solids) to determine the specific response of each species to nutrient additions. 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. 

Shrimp biofloc solids were collected from intensive, minimal-exchange culture systems for Litopenaeus vannemei (Pacific white shrimp) at the University of Southern Mississippi’s Thad Cochran Marine Aquaculture Center. The formation of biofloc particles (composed of micro-organisms, feed, detritus, and feces) occur in these systems as a result of the high nutrient inputs required for the high stocking density.  Management of the concentration of biofloc particles in shrimp production has been shown to increase shrimp production, so settlement chambers were used to filter solid particles from the liquid effluent (Ray et al. 2011).  These chambers were drained periodically, and the removed solids were allowed to dewater in a greenhouse.  Once dried, the solids were collected and stored at -20˚C prior to the experiment.  Before being used, biofloc solids were ground, 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 November to March 2011.  Six 4-inch pots of S. alterniflora and J. roemerianus were placed in shallow nursery trays for a total of 24 trays per species (6 pots x 24 trays = 144 pots/species).  Trays were randomly placed in a 10 x 12 foot greenhouse and sub-irrigated daily (Figure 1).   For each species, eight trays were randomly assigned one of the following treatments (8 trays x 3 treatments = 24 trays/species): (1) Control treatment, with the addition of 30-ml water, (2) Miracle-Gro™ treatment, with the addition of 30-ml Miracle-Gro™ 20-20-20 dissolved in water (74-g Miracle-Gro™ / 5.5-L water), and (3) Shrimp Solids treatment, with the addition 7-g of ground dried shrimp solids and 30-mL water (Figure 1).  Dried shrimp solids had a total nitrogen content of 6.8% (0.6% NO3­- and 6.2% NH4+), 1.1% total phosphorous content, pH of 9.2, and salinity of 30 ppt.  Miracle-Gro™ had 20% nitrogen (primarily in the form of urea (CH4N2O) and ammonium phosphate) and 20% phosphorous (ammonium phosphate).  Miracle-Gro™ was prepared at approximately five times the normal concentration to reflect the nitrogen content of shrimp solids (~112 mg/L nitrogen per dose for both Miracle-Gro™ and shrimp solids).  Treatment additions were made weekly from November 4 to December 9, 2011, for a total of six weeks (dosing period), and plants remained in the greenhouse until March 20 2011 (12 weeks) to allow for incorporation of nutrients into plant biomass during spring growth.

Figure 1: Graphical representation of experimental design.  Trays of Spartina alterniflora (SA) and Juncus roemerianus (JR) were randomly placed on top and bottom (BOT) shelves in a greenhouse and assigned one of three treatments: (1) Control (C) treatment, (2) Miracle-Gro™ (MG) treatment, and (3) Shrimp Solids (SS) treatment.  Each tray had six 4-inch pots with several plants per pot (inset).

Results

There was a clear fertilization effect of both nutrient additions (i.e., Miracle-Gro™ and shrimp biofloc solids) on S. alterniflora and J. roemerianus.  For both species, fertilized plants had greater above-ground biomass, tissue nutrients, and chlorophyll compared to control plants, consistent with other studies on the effect of nutrient addition on salt marsh plants.  For many of the measured variables, there was no significant difference between Miracle-Gro and Shrimp Solids treatments, indicating some similarity between the two nutrient sources and their effect on plants.  Both the commercial fertilizer and shrimp biofloc solids had the majority of nitrogen present as ammonium, which is easily assimilated and incorporated into plant tissues.  However, the two species differed in the degree of response to the source of nutrient addition.  Spartina alterniflora plants had a stronger response to the commercial fertilizer compared to the shrimp biofloc solids while J. roemerianus plants responded similarly (with the exception of below-ground biomass) to both nutrient sources.

Figure 2: Summary of the effect of treatment (i.e., Control, Miracle-Gro™, and Shrimp Solids) on plant biomass, nutrient concentration, and chlorophyll content for Spartina alterniflora and Juncus roemerianus plants at the end of the dosing period (week 6) and in the response period (week 18).  Larger plants/roots indicate more relative biomass, text size of percent nitrogen (%N) and percent phosphorous (%P) indicates the relative content per treatment, and the greenness of plants represents greater relative chlorophyll content.

Conclusions

The results of the present study suggest that both agricultural and aquaculture sources of nutrient additions will impact salt marsh plant biomass allocation and will likely have a negative impact on the salt marsh community, resulting in changes in plant zonation and species diversity.  Spartina alterniflora showed a stronger response to commercial fertilizer nutrient additions compared to shrimp solids while J. roemerianus responded similarly to both nutrient sources.  However, the increased belowground biomass in J. roemerianus plants in the shrimp solids treatment suggests that the lower phosphorous levels of shrimp solids compared to commercial fertilizers may promote belowground growth in marshes limited by phosphorous.  Based on these results, it is likely that the addition of nutrients from both sources will reduce belowground competition for nutrients (which functioned to competitively exclude nitrogen limited species such as S. alterniflora) and facilitates the invasion of the faster-growing S. alterniflora into zones dominated by J. roemerianus.