Increased biomass of free-living marine nematodes may be indicative of disturbances in the ecosystem of the San Antonio Bay

Benthic organisms make a signifi cant contribution to the regulation of carbon, nitrogen and sulfi de cycles, water column processes, the distribution and destination of pollutants, secondary production and transport and stability of sediments [9]. In the coastal zones the meiofauna biomass is smaller than the macrofauna biomass. However, due to its higher renewal rates (B/W), meiofauna plays a proportionally larger role in energy transfer. Therefore, the metabolic requirements of meiofauna (secondary production and respiration) could reach values even higher than those recorded for the macrofauna, particularly in environments where the ratio of macrofauna: meiofauna biomass approaches or is less than 5:1 [10,11]. From an ecological perspective and according to their adaptive and behavioral characteristics as r-k strategists, families of nematodes can be ordered on a scale as colonizers (corresponding to type r strategists), persistent (corresponding to type k strategists) and those with behaviors intermediates [12]. The maturity index is used as an effective measure for the evaluation and quantifi cation of the impact of soil contamination in the nematode community.

Nematodes are the most diverse and numerically dominant metazoans in aquatic ecosystem, and, because of their rare ability to survive in extremely polluted conditions, they ase ususally the only persistent taxon in heavily polluted/disturbed habitats [8].
Benthic organisms make a signifi cant contribution to the regulation of carbon, nitrogen and sulfi de cycles, water column processes, the distribution and destination of pollutants, secondary production and transport and stability of sediments [9]. In the coastal zones the meiofauna biomass is smaller than the macrofauna biomass. However, due to its higher renewal rates (B/W), meiofauna plays a proportionally larger role in energy transfer. Therefore, the metabolic requirements of meiofauna (secondary production and respiration) could reach values even higher than those recorded for the macrofauna, particularly in environments where the ratio of macrofauna: meiofauna biomass approaches or is less than 5:1 [10,11]. From an ecological perspective and according to their adaptive and behavioral characteristics as r-k strategists, families of nematodes can be ordered on a scale as colonizers (corresponding to type r strategists), persistent (corresponding to type k strategists) and those with behaviors intermediates [12]. The maturity index is used as an effective measure for the evaluation and quantifi cation of the impact of soil contamination in the nematode community. 200 m in the center of the bay. Fishing is carried out in that city and in the port of San Antonio Este, one of its main activities is the export of products from the Río Negro valley.
For this work we chose an urbanized beach ("Ciudad") where fi shing activities are carried out and the products exported from the Black River Valley and two pristine beaches ("Baliza Camino" and "Banco Perdices").
The objective of this work was to carry out an ecological study of the free-living marine nematode communities of the salt marshes of San Antonio bay, in order to be able to evaluate the volumetric biomass could be indicative of the disturbances produced by the man.
Thus, two main questions have been addressed: is the volumetric biomass a useful tool to detect the effects of a human event? and the volumetric biomass differs between the three sites studied in San Antonio bay (Río Negro)?

Sample collection and treatment
The samples were collected with a cylindrical Plexiglas corer, 10cm high and 2.9cm in diameter. They were preserved in 5% formaldehyde in fi ltered seawater, sieved through both 500μm and 50μm mesh sieves. The nematodes present on the 50 μm were separated by the sediments by means of LUDOX TM [13]. Then, 200 specimens were separated from each sample, which were mounted in glycerin between slide and coverslip and sealed with CANADAX resin. Then, we proceeded to the identifi cation to species level.
The maturity index (IM) can be calculated for each site and level using the C-P values of the species found using the formula of Bongers, et al., 1991 [14]: IM=∑ v x f Where v is the C-P value of each taxon and f is the frequency taxon.
The length (L) and width (W) of each of the nematodes were measured. To determine the biomass of the different groups, the Andrassy [15], formula was used: Where V is the volume in ml, C is a constant according to each taxon, L is the organism length and W is the width. The value of the constant C was obtained from Feller and Warwick [16] , where C=530. The volume was converted to wet weight by multiplying it by the specifi c gravity of the organism (1.13 For each sample the following environmental variables were measured: 1) dissolved oxygen (O 2, mg/l), water salinity (S, g/l) and temperature (T,°C) (they were measured in situ with a multiparameter equipment), 2) the depth of the anoxic layer (cm), 3) penetrability (P, cm), 4) organic matter (MO), 5) grain size classifi cation of sediment and 6) heavy metals.
The samples of heavy metals (Zn, Pb, and Cu) were analyzed by the chemistry service of the "Centro Nacional Patagónico" (CENPAT).
For the characterization of the sediment each sample was weighed, then all of them were dried in a stove at 60°C, after they were sifted and weighed again. The difference between the original weights after sifted was analyzed to determine the Fine Fraction (FF). The samples of organic matter were dried in a stove at 60°C and then with a mortar were homogenized.
From each sample a subsample was taken which was placed in a muffl e at 450°C. Subsequently the weight of each subsample was recorded and the value of organic matter was calculated by weight difference [17].

Analysis of nematode distribution patterns
All of the multivariate analyses were performed with the PRIMER v6 software [18].
Cluster analysis derived from Bray-Curtis similarity matrices (fourth-root transformed) was used to view spatial differences in the biomass of the community of nematodes.
To evaluate the differences between the samples, the one-way ANOSIM test and the 999 permutations were performed. A Rglobal value is obtained from 0 (when there are no differences between samples) to 1 (when the samples differ from each other) and a probability p, which indicates the signifi cance of the analysis.
Two methodologies were used to analyze the environmental parameters: 1) the main component analysis (PCA) [19]. For this, a matrix was constructed in which the data were transformed using Log(x+1) and then normalized. A graph was obtained that showed the relation between the structure of nematode associations in biomass and the environmental parameters; 2) the BIOENV subroutine, which defi nes the set of environmental variables that best explain the biotic structure.
Individuals were classifi ed within C-P classes and the maturity index was calculated for each site and mesolitoral level. Taking into account the level factor of the mesolitoral, the ANOSIM test found signifi cant differences (p=0.001;

Samples taken in San
Rglobal=0.736). The differences were refl ected between the upper and the middle level, and between the upper and lower levels; while they did not do it between the middle and lower levels.
To determine which species generate dissimilarity between sites based on volumetric biomass, those that contributed up to 60% were considered. The BEST analysis that relates the environmental variables to the biomass data showed low correlation values, the highest value being the salinity variable (S), which was 0.404g/l (Table 1).   With respect to the C-P values, in San Antonio bay the strategists type C-P 3(82%) were the most dominant, followed by strategists C-P 4(10%) and C-P 2(8%). No individuals with strategy type C-P 1 and C-P 5 were found.

Discussion and conclusion
The bay of San Antonio was characterized by high salinity.
The bay showed high salinity values in two of the three sites (~50g/l), "Ciudad" (Site A) and "Banco Perdices" (Site C) and very high in "Baliza Camino" (Site B), exceeding 100g/l ( Table   2).
The The values of C-P 2 and C-P 3 were found most frequently in the "Ciudad". It is probable that the nematodes found in this site have developed a certain tolerance to these concentrations of metals, and therefore have the highest volumetric biomass.
The middle and lower levels did not present differences in relation to volumetric biomass. Although the metal levels were higher than the rest of the sites, the effect was not notorious.
At the lower level the highest amount of organic matter was observed. The process of eutrophication, as a result of port activity [21] and sewage discharges was notorious in this levels.
Site B presented the highest salinity of the 3 sites, together with the lowest volumetric biomass. The number of species was low, standing out as the ones that contributed most to the biomass: Paraethmolaimus dahli, Diplolaimella gerlachi and Paracyatholaimus sp. According to Heip, et al., [22], the species that are in a habitat with high salinity are well adapted, and observed that some species survived a salinity of 123g/l, such as  showed great tolerance to heavy metals and high salinity, which is why they can be considered as indicator species.