Modelos Metáforas sobre la función del intérprete de lengua de signos

Understanding the spatial and temporal variations in communities is important to determine the pattern of distribution, abundance, maintenance of species diversity, and stability of communities (Koenig 1999; Ellingsen 2002; Thrush et al. 2010; Moens et al. 2013).

3.3.3.1. Temporal patterns

Marine nematodes exhibit temporal fluctuations on the scale of days to years (Alongi 1990b; Gourbault et al. 1998; Nicholas 2001; Riera et al. 2011b; Maria et al. 2013a; Ramalho et al. 2014), even though some studies have observed a lack of any temporal pattern (Warwick and Buchanan 1971; Juario 1975; Liu et al. 2007). Information on temporal variation in marine nematode assemblages has mostly focused on seasonal variations (Boaden and Platt 1971; McIntyre and Murison 1973; Platt 1977; McLachlan 1978; Blome 1982; Sharma and Webster 1983; Gourbault et al. 1998; Nicholas and Hodda 1999; Nicholas 2001; Albuquerque et al. 2007; Liu et al. 2008; Venekey et al. 2014a), whereas longer-term variation has received only limited attention (Coull 1985, 1986; Eskin and Coull 1987; Li et al. 1996; Riera et al. 2011a). Below, we mention the main causes for temporal fluctuations of marine nematodes.

Temperature is an important abiotic factor in marine environments. Even slight temperature changes can impact reproductive and metabolic activity (Moens and Vincx 2000). It can affect nematode abundances directly, e.g. via dehydration and/or effects on reproduction, and indirectly as well, e.g. by controlling the growth of food items such as bacteria and diatoms (Harris 1972a) and/or by affecting the depth of the redox discontinuity layer in sediments (Dye 1983). Temperature regime can also have a substantial effect on the interspecific interactions and life-history characteristics of marine nematodes (De Meester et al. 2015b; De Meester et al. 2015c).

Other climatic factors such as precipitation can also cause seasonal fluctuations in nematode assemblages. Some studies in tropical regions have, for instance, indicated that nematode density changes are affected by rain cycles (Pattnaik and Lakshmana Rao 1990; Ingole and Parulekar 1998; Venekey et al. 2014b). On a much shorter temporal scale, short episodes of rainfall, such as a heavy shower on an exposed intertidal flat, may cause vertical migrations of nematodes in sediments (Steyaert et al., 2001).

33 Salinity is another important factor affecting both the temporal and spatial distribution of marine nematodes. From a temporal perspective, salinity fluctuations are linked with seasonal cycles in precipitation (Paranhos and Mayr 1993). Within sandy beach habitats, there can also be considerable short-term variability in salinity, both in direct relation to the tides and indirectly through episodic events such as the above-mentioned episodes of heavy rainfall during low-tide exposure (Steyaert et al., 2001).

Food availability is another driver of temporal as well as spatial variation in nematode assemblages (Heip et al. 1985; Moens et al. 2013). For instance, the densities of certain nematode feeding guilds have been shown to be related to variations in the abundances of the food sources of those groups (Austen and Warwick 1995; Kendall et al. 1995; Ólafsson and Elmgren 1997).

3.3.3.2. Spatial patterns

Spatial distribution of marine nematodes can be viewed at large and small scales. Several factors have been proposed to explain these distributions, including physico-chemical (e.g. temperature, salinity, mean grain size of sediment and dissolved oxygen concentration) (Steyaert et al. 2003; Hourston et al. 2005; Nozais et al. 2005; Adão et al. 2009) as well as biological factors such as food quality and quantity, predator/competition impacts and reproductive behaviour (Montagna et al. 1983; Rudnick et al. 1985; Moens et al. 1999; Gallucci et al. 2005; dos Santos and Moens 2011; Maria et al. 2012; Urban-Malinga et al. 2016). It is often suggested that physico-chemical factors typically determine macro-scale (e.g. kilometre scale) patterns, whilst biological factors cause micro-scale (e.g. (sub)metre scale) heterogeneity (Heip et al. 1985; Moens et al. 2013). Marine nematodes are generally influenced by these complex and interacting physical and biological processes, leading to variation in their distribution at different spatial and temporal scales.

Salinity is a key factor with strong significance on nematode distribution, especially in estuarine habitats (Adão et al. 2009; Chen et al. 2012). A strong relationship between salinity gradients and meiofaunal/nematode assemblage structure has been reported (Austen and Warwick 1989; Barnes et al. 2008). Salinity has also been correlated with the distribution of particular nematode feeding types in estuarine habitats (Austen 1989). At high salinities, selective deposit feeders and epigrowth feeders tend to dominate, while at lower salinities, omnivores and non-selective deposit feeders often do so. These trends perhaps relate to the

34 availability of organic matter in different reaches of an estuary. In addition to gradients in average salinity, the local range of salinity fluctuations varies along estuaries but also, for instance, across the intertidal zone; ranges of daily salinity fluctuations may have yet more pronounced effects on marine/estuarine organisms than average values per se (Kaiser and Attrill 2011). To what extent elevated mean salinity, as in the PG area, affects nematode abundance, diversity and assemblage composition is not properly documented.

As marine nematodes live constantly in the sediment, any changes in sediment characteristics (grain size, grain shape, sorting and sediment pore space) may have an effect on their assemblage structure (Giere 2009). Herman (1982) and Boyd et al. (2000b) reported that nematodes are more sensitive to shifts in sediment structure than macrofauna. For example, it has been frequently reported that coarser sediments have higher nematode species richness and diversity than finer sediments (Heip and Decraemer 1974; Steyaert et al. 1999; Vanaverbeke et al. 2002; Semprucci et al. 2010; Vanaverbeke et al. 2011). Similarly, nematode assemblage composition also changes according to the granulometric properties of the sediment (Wieser 1959; Heip et al. 1985; Vincx et al. 1990; Vanaverbeke et al. 2002; Vanaverbeke et al. 2011; Fonseca et al. 2014). In addition, nematodes from sandy habitats are often more slender as they have to move through the sediment, whereas nematodes from muddy habitats are generally more robust for burrowing through the sediment (Moens et al., (2013). Gheskiere et al. (2004) also found that grain size explained the horizontal nematode distribution at a Belgian beach (De Panne) (see 3.4.1).

Sediment characteristics also determine other aspects of the environment of the sediment, such as organic matter availability (Coull 1985; Danovaro and Gambi 2002). Generally, clay and silt substrata retain more organic matter than sand. It has been reported that the content of organic matter can partly explain the spatial patterns of distribution of free-living nematodes in some marine habitats (Ólafsson and Elmgren 1997; Schratzberger et al. 2006). (Sajan et al. 2010) reported that average biomass and density of nematodes were higher in silt/clay substrata than in sandy and mixed sand. The authors stated that fine particles may hold more labile organic matter and this may cause higher biomass and density of nematodes, supporting the general trend of higher nematode abundances found in finer sands (Adão et al. 2009). Comparison of sheltered versus exposed sandy beaches further indicates that meiofauna densities at the sheltered beaches are usually higher compared with exposed beaches (Ólafsson 1991; Gheskiere et al. 2002; Corgosinho et al. 2003; Urban-Malinga et al. 2004; Gheskiere et al. 2005; Hourston et al. 2005). In sheltered beaches, the sediment stability increases organic matter accumulation. However, some studies (Calles Procel et al. 2005)

35 have reported contrasting trends, namely highest meiofauna densities in exposed beaches and lower densities in sheltered beaches, but these results were probably related more to anthropogenic impacts than to real granulometric effects. Organic matter availability, in turn, is an important determinant of sediment oxygenation because it stimulates microbial growth, which consumes most of the oxygen (Bickford 1996; Kristensen 2000; Steyaert et al. 2007). In addition, organic matter can bind pollutants, thus affecting their retention in sediments and availability to consumers (Philippe and Schaumann 2014; Mazzei and Piccolo 2015).

Pollution is indeed another important abiotic factor affecting the distribution pattern of marine nematodes; effects depend on pollutant type, exposure level and field conditions. Pollutants influence marine nematodes by changes in abundance and diversity, trophic group composition, reproductive ability etc. (Nanajkar and Ingole 2010; Balsamo et al. 2012; Kang et al. 2014).

Furthermore, marine nematode distribution patterns can result from biological interactions. Biological interactions between nematodes and macrofauna can occur and influence nematode distribution (Van Colen et al. 2009; Braeckman et al. 2011; Maria et al. 2011b; Van Colen et al. 2012; Urban-Malinga et al. 2016). According to Mirto et al. (2000), for instance, mussels induce changes in sediment characteristics, organic matter quantity and quality, and depth of oxygen penetration in the sediment.

Moreover, competition, both among individuals within a species and among species, can also play a major role in limiting faunal abundances and distribution (Gray and Elliott 2009; Moens et al. 2013). Predation among nematodes can also be responsible for controlling nematode densities and perhaps also diversity (Gallucci et al. 2005; dos Santos and Moens 2011).

Aggregative small-scale horizontal distribution (at scales of meters or less) of meiofauna is a well documented phenomenon (Ólafsson 1992; Blome et al. 1999; Giere 2009; Maria et al. 2013b; Urban-Malinga 2014). The causative factors for patchiness are multifactorial: biological factors, such as reproductive activities, predation and availability of, and competition for food have been reported as main drivers (Giere 2009). Nematodes are highly influenced by small-scale patches of food and by disturbance, both of which create microhabitats in space and time. The resulting spatial patchiness may be defined at the scales of cms (Findlay 1981; Blanchard 1990; Ólafsson 1992; Sandulli and Pinckney 1999), and due to such patchy distribution pattern, meiofauna density and assemblage composition may fluctuate over distances of a few centimeters. The spatial autocorrelation between patches and

36 patch sizes of meiofauna and of microphytobenthos suggests that at least in intertidal sediments, variations in food resources may be the principal determinant of meiofauna small- scale patchiness (Findlay, 1981; Blanchard, 1990).