Space–Time Patterns of Co-Variation of Biodiversity and Primary Production in Phytoplankton Guilds of Coastal Marine Environments
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AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 13: 489–506 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/aqc.590 Space–time patterns of co-variation of biodiversity and primary production in phytoplankton guilds of coastal marine environments MARIA ROSARIA VADRUCCIa*, FABIO VIGNESb, ANNITA FIOCCAa, ALBERTO BASSETa, IMMACOLATA SANTARPIAb, GIAN CARLO CARRADAb, MARINA CABRINIc and SERENA FONDA UMANIc a Dipartimento di Science e Tecnologie Biologiche e Ambientali, Universita" di Lecce, Lecce, Italy b Dipartimento di Zoologia, Universita" degli Studi Federico II di Napoli, Napes, Italy c Dipartimento di Biologia, Universita" degli Studi di Trieste, Trieste, Italy ABSTRACT 1. The relevance of biodiversity to ecosystem processes is a major topic in ecology. Here, we analyse the relationship between biodiversity and productivity of the nano- and micro- phytoplankton guilds in coastal marine ecosystems. 2. The patterns of variation of species richness, diversity and primary productivity (as 14C assimilation) were studied in two marine areas: a eutrophic–mesotrophic area beside the River Po delta (northern Adriatic) and an oligotrophic area around the Salento peninsula (southern Adriatic– Ionian). The study was carried out at 23 sites in the northern area and at 45 sites in the southern area. Sites were arranged on expected spatial and temporal gradients of primary productivity variation, according to distance from the coast, optical depths and seasonal period. 3. 167 taxa were identified in the northern area and 153 taxa in the southern area. In both areas, the taxonomic composition of the nano- and micro-phytoplankton guilds exhibited greater temporal than spatial variation. The latter was much higher in the southern area than in the northern area (average dissimilarity between stations being 70:7 Æ 0:8% and 44:7 Æ 4:2% respectively). 4. Primary productivity varied in space and time on the gradients considered. Phytoplankton species richness and diversity exhibited significant patterns of variation in space and time; overall, these were inversely related to the primary productivity patterns in the northern area, whereas they were directly related in the southern area. 5. The small individual size and the high turnover rate of phytoplankton are likely to underlie the observed relationships, which emphasized a threshold response to nutrient enrichment in agreement with the ‘paradox of enrichment’. Under resource enrichment conditions, the high turnover of producers leads to hierarchical partitioning of the available resources with an increasing dominance of a few species. Therefore, the relationship observed here seems likely to be explained by the complementarity hypothesis. Copyright # 2003 John Wiley & Sons, Ltd. *Correspondence to: Maria Rosaria Vadrucci, Dipartimento di Science e Tecnologie Biologiche e Ambientali, Universita" degli Studi di Lecce, Via Provinciale Lecce–Monteroni, 73100 Lecce, Italy. E-mail: [email protected] Copyright # 2003 John Wiley & Sons, Ltd. Received 5 May 2002 Accepted 4 March 2003 490 M. R. VADRUCCI ET AL. KEY WORDS: phytoplankton; primary production; species diversity; coastal marine environment; spatial heterogeneity, Adriatic Sea; Ionian Sea INTRODUCTION In recent years, many ecosystems have suffered from a loss in biodiversity as a result of the expansion of anthropogenic activity. The implications of this activity on the ecosystem functions and the services that ecosystems perform are receiving increasing attention (Schulze and Mooney, 1994; Jones and Lawton, 1995; Schlaapfer. et al., 1999). For this purpose, studies of the relationship between biodiversity and ecosystem functioning have made rapid progress in the past decade (Purvis and Hector, 2000). These have tried to determine whether biodiversity influences ecosystem functioning and, if so, what the resulting effects of human-driven biodiversity loss could be on this. Moreover, the relative importance of the argument for the conservation of species has been widely discussed (Bond and Chase, 2002). Different paradigms have arisen with regard to the diversity–function issue (Naeem et al., 1994; Tilman et al., 1996; Hooper and Vitousek, 1998; Laakso and Setaal. aa,. 1999), and this has hampered government action in terms of implementing strategies for the protection of biological diversity. The theory of niche and competition has dealt, albeit indirectly, with this relationship since the 1960s (MacArthur and Levin, 1967), pointing to direct and reciprocal links between biodiversity and productivity. The hypothesis of a direct relationship between species diversity and productivity is based on the assumption that the inter-specific differences in the use of resources by plants allow guilds characterized by greater diversity to make more efficient use of limiting resources (McNaughton, 1994; Tilman et al., 1996). This approach, based on niche partitioning, has recently been defined as the complementarity hypothesis (Fridley, 2001). A direct relationship between biodiversity and productivity has been observed primarily in terrestrial ecosystems in terms of both taxonomic (Tilman et al., 1996; Hector et al., 1999) and functional diversity (Tilman et al., 1997a; Hector et al., 1999). This emerges from field studies (Pianka, 1966; Currie and Paquin, 1987; Currie, 1991), microcosm experiments with manipulation of the number of species (Naeem et al., 1994) and mesocosm experiments (Tilman et al., 1996; Hector et al., 1999). A direct relationship between biodiversity and productivity is also supported by mathematical modelling of non-equilibrium systems, as a probabilistic effect of increasing the number of species (i.e. sampling effect; Tilman et al., 1997b; Loreau, 1998). On the other hand, the likelihood of a direct relationship between diversity and productivity seems to depend on the spatial heterogeneity of the environment (Jansen and Mulder, 1999; Fridley, 2001) and on nutrient enrichment (‘paradox of enrichment’; Rosenzweig, 1971), which limit the potential for resources partitioning among co-existing species. Indeed, the mechanism by which resources are partitioned assumes that there is heterogeneity at the scale of species response (Bell and Lechowicz, 1994). In marine environments with small producers and a high turnover, cases of nutrient enrichment have consistently been associated with increases in primary production and reductions in the number of species and diversity of producers (Margalef, 1978; Huston, 1994; Valiela, 1995). The form and the meaning of the diversity–productivity relationship are thus still controversial and seem to depend on the mechanisms that underlie the organization of communities and the evolution of biodiversity (Jansen and Mulder, 1999; Fridley, 2001), and which result in the differences between various types of ecosystem. Marine coastal ecosystems, by virtue of their highly dynamic nature and the temporal and spatial heterogeneity of their main physico-chemical characteristics on well-defined gradients of variation, Copyright # 2003 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 13: 489–506 (2003) DIVERSITY–PRODUCTIVITY RELATIONSHIP IN PHYTOPLANKTON GUILDS 491 constitute natural laboratories in which the diversity–productivity relationship can be studied on short time scales, suitable for experimental analysis. The aim of this study is to analyse the relationship between diversity of phytoplankton producer guilds and primary production in two marine coastal areas, by verifying: 1. the existence and form of the diversity–productivity relationship in marine coastal environments; 2. the role of nutrient enrichment and spatial heterogeneity in taxonomic abundance, diversity and evenness of phytoplankton guilds, and in the relations of these with primary production; and 3. the relevance of complementarity and the sampling effect in determining the diversity–productivity relationship in marine coastal environments. To this end, the structural characteristics and primary production of nano- and micro-phytoplankton guilds, taxonomically identifiable using Utermoohl’s. method, were studied on gradients of productivity that were identifiable a priori, in two marine areas of the Adriatic–Ionian: (i) the northern Adriatic, with high trophic levels (Fonda Umani et al., 1992; Zoppini et al., 1995; Alberighi et al., 1996); (ii) the southern Adriatic–Ionian, characterized by low trophic levels (Fonda Umani et al., 1992; Rabitti et al., 1994; Socal et al., 1999; Zavatarelli et al., 2000). The original data, on which this study is based, were gathered during the PRISMA II and INTERREG II Italy–Greece oceanographic cruises, for the northern Adriatic and southern Adriatic–Ionian areas respectively, with methods that were similar, though not identical, and with different sampling times. In the present study, the two sets of data are analysed separately and the comparison between the two areas concerns only the existence or otherwise of the diversity–productivity relationship and, where appropriate, its shape. MATERIALS AND METHODS Sample collection The data relating to the structural and functional characteristics of the nano- and micro-phytoplankton guilds were gathered in the northern Adriatic in the course of four oceanographic cruises pertaining to sub- project 2 of the PRISMA II project and in the southern Adriatic–Ionian in the course of three oceanographic cruises pertaining to the INTERREG II Italy–Greece project. In the