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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002

http://dx.doi.org/10.4067/S0717-71782002030100049 

Temporal Dynamics of the
Macroinvertebrate Communities
Associated with the Mangrove and
Sand Ecosystems at Piangüita
(Bay of Buenaventura, Colombian
Pacific) from October 1999
to March 2001

Leonardo Herrera

Department of Marine Biology, Universidad del Valle;
A.A 25360, Santiago de Cali, Colombia,
E-mail: leoheroz@libertad.univalle.edu.co

Introduction & Objectives

Piangüita (30 49´ 30´´ N; 770 12´ 32´´ W), is located North West of Buenaventura on the Colombian Pacific coast. The macroinvertebrate fauna associated with the soft bottom mangrove and sand zones of this area was observed from October 1999 to March 2001, and records of ecological indices were made. Sampling was carried out in four places distributed according to sediment: Zones A and B represent mud bottom, although the latter is more consolidated, and both zones are covered by a degree of vegetable material, produced by leaves falling from mangrove trees. Zone C has a mud-sand floor with organic material carried from a brook, and zone D has a sandy floor, and is bounded by the high tide line.

Mangrove and sand ecosystems have a great economic and ecological importance; the main purpose of this monitoring was to find out how the communities of macroinvertebrates associated with them are developing, and how they will respond to climatic changes. There are few works in this area (Bay of Buenaventura), and a more general objective was to build a database to support future investigations.

Results

Composition

515 individuals were collected in 13 months: 27 species and 7 phyla. The dominant group was Polychaeta with 238 individuals (46.21%) and 7 species (25.92%); other important groups were bivalves with 98 individuals (19.03%) and 7 species and decapods with 108 individuals (21.00%) and 5 species (18.52%); other groups such as Gastropoda, Nemertina, Echiurida and Amphipoda were present in low numbers.

Sampling zones showed similar numbers of species, which oscillated among 13 and 16; Abundance (of individuals) fluctuate between 53.33 ind/m3 and 1520 ind/m3 in zone A; 106.67 ind/m3 and 1280 ind/m3 at B; 26.67 and 1280 ind/m3 at C, and between 80.0 and 2240 ind/m3at zone D. As expected, some species were found in more than one zone, these were: Uca tenuipedis, Uca pigmaea, Platynereis sp., Laeonereis sp., Talitrus sp., Iphigenia altior and Notomastus sp. In order to analyse results, records were grouped in two ways: common species and not common species; the first group represents 30% of all species and ws made up of species found in at least eight samples. The second group represents the remaining 70%.

Temporal dynamics

The community included established populations of Notomastus sp, Glycera sp, Platynereis sp, Laeonereis sp and two species of Uca; these were found to have a steady presence, but fluctuating abundance. Iphigenia altior and Cardita affinis were present for long periods with a low and stable abundance. Finally, we noted an increase in the abundance of Glycera sp in the last three samples, presenting similar values as all other previous samples. Some species presented explosive values in abundance in different samples but disappeared in the others.

The four sampling zones presented constant and low values of diversity (Shannon-Weaver index, H') and number of abundant species (Hill's number, N1) in the four final months; on the other hand, equitability (Pielou index, J') showed high values, except with zero values. The number of species or richness (Hill's number, N0 or S) fluctuated between 1 and 9, being on average close to 4 sp/zone (Fig. 1)


Fig. 1 Total abundance of macroinvertebrates at Pianguita (Oct/99-Mar/01). It shows values with tendency to a similar tendencies; values outside of this tendency represent Population events of different species. Zone A: rhombus, B: squares, C: triangles and D: circles.

Physical and chemical parameters presented normal values compared with historical records from Piangüita (from reports of the IDEAM, Instituto de Estudios Ambientales y Metereológicos de Colombia), this is associated with regular climatic conditions.

Discussion

The most significant events during the monitoring time were the low values in Specific Ecological Richness and Abundance found in the macrofauna communities associated with Piangüita´s mangrove and sandy beach, although an increase in this index was observed in the last four samples. More data are necessary to explain this trend. We believe that dominant species reestablish their populations with no entry of new individuals to the ecosystems, possibly by constant disturbance caused by tidal changes. This also explains why Diversity and Richness values were similar, being present with slow increases associated with the late stages of succession.

High Equitability values suggests that populations of these communities were in the process of growing and that neither population developed in a dominant way. Zero values obtained for the Pielou index were not interpreted as a real domain of species; we think that these values are correlated to specific richness, which was, as we said, normally low during the monitoring time, especially in March 2000, when a maximum of two species were found.

The number of Abundant species (N1) regularly showed records close to the maximum value (allowed by H´ value). This suggests that each population increased independently, meaning that resources were sufficient to feed established populations (this idea is supported by the high equitability values found).

Conclusions

Ecosystems like mangrove, which are continuously disturbed, do not allow populations to establish quickly. The macroinvertebrate community associated with soft bottom zones monitored in Piangüita was no exception. Ecological indices obtained show established populations with fluctuations in abundance and surging in addition to spontaneous populations with catastrophic comportment which increased to high values of density in different months and disappeared. This, together with the fact that environmental conditions were normal during the monitoring period, lead to the conclusion that the fluctuation shown by community indices resulted from biological population events.

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