Diatom of the Month – June 2017: Fragilaria synegrotesca

by Nick Schulte*

I think Fragilaria synegrotesca is a cute diatom. Although long and lanky (nothing wrong with that!), F. synegrotesca has an adorable, sometimes very slight, potbelly (Fig. 1). 

  

              

Fig. 1. a) Live frustules in a rosette colony (http://fcelter.fiu.edu/data/database/diatom/index.htm?species=3568) 

b) Fragilaria synegrotesca in valve view (Schulte 2014).

Now, some boring diatomist (e.g., me) might describe that little bump in the middle right as “a unilaterally expanded, hyaline central margin” and that’s accurate enough. But I also like to think of it as F. synegrotesca’s belly pooch. It brings to my mind the potbellies of seahorses, pigs, puppies and toddlers, and it seems very boop-able.

But let’s move past the physical attributes of this diatom, as the allure of this species is in its “actions”. Fragilaria synegrotesca has so far only been reported from karstic wetlands of the Caribbean and is most well-known from the Florida Everglades. In the Everglades, F. synegrotesca is nearly ubiquitous (Fig. 2), and it’s one of the five most abundant species in the calcareous periphyton mats in the nutrient-poor freshwater marshes (Gaiser et al. 2006). 

Fig. 2. Relative abundance (%) of F. synegrotesca across the Everglades (data from the Comprehensive Everglades Restoration Plan Monitoring and Assessment Plan).

A major issue in Everglades restoration is getting the amounts of water and nutrients that enter this wetland right. Every winter/spring (the “dry season”), sloughs and inundated prairies often dry down. This happens more often and more severely now than in the “natural” pre-drainage state in many sites. But water managers (e.g., the South Florida Water Management District) can’t just send water through the marshes unless it’s “clean” (e.g., low in phosphorus), so as not to harm organisms that are adapted to this wetland’s low nutrient waters. So, Everglades restoration is between a bit of a rock and a hard place: we need to deliver more water to help the organisms that need high water (and can’t handle severe dry-down – e.g., many fish), but not at the expense of the organisms that can’t handle high nutrients in the water (e.g., some grasses and sedges).

Fragilaria synegrotesca is one of those organisms that doesn’t like to be dried out (Gottlieb et al. 2005, Lee et al. 2013), and its preference for being wet makes it a potentially “reliable indicator of the absence of periodic drying” in the ‘Glades (Gaiser et al. 2011). We can therefore use the abundance of this species (alongside other indicators) to measure the effects on biodiversity that potential reduced water flow might have upon different locations. This information can then inform decisions on how much water should be sent where and when – all key questions in Everglades restoration.

Unlike its freshwater-loving, high nutrient-hating buddies Brachysira microcephala, Encyonema evergladianum, and Mastogloia calcarea (let’s call them the “Fresh Diatoms of Belle Glades,” or “Freshies” for short), F. synegrotesca can also live comfortably in moderate phosphorus (P) concentrations and slightly salty water (“oligohaline”).

So, we can think of F. synegrotesca as that close friend that is too cool for us sometimes and likes to hang out with hipper, more indulgent folks.

And if this diatom is found in relatively high abundance in the absence of the Freshies, we know that area might be getting a little too phosphorus-y and/or salty than is normal. Now, there are some regions of the Everglades where finding F. synegrotesca in enriched or salty places is normal, but by now we know which places are “normally” enriched/salty and which are not. So, if we see this species hanging out with the Salty Boys or the +P Posse in the good side of town (i.e., a normally freshwater, low nutrient place), we know something’s about to go down. In this way, I guess F. synegrotesca is also like that sweet suburban kid who gets caught up in the wrong crowd, and we’d rather see it back at home with the Freshies.

But here is some science to back up these potentially confusing analogies. In the Everglades, the total phosphorus (TP) optimum of F. synegrotesca is 270±202 µg P g^-1 periphtyon (Gaiser et al. 2006), and this species has been designated as an indicator of high TP (La Hée and Gaiser 2012). Compare that to oligotrophic, freshwater indicators (B. microcephala, E. evergladianum, and M. calcarea) that have a mean TP optimum of 159 µg g^-1 (Gaiser et al. 2006). Our diatom of the month also has a salinity optimum and tolerance of 5±7.3 ppt (parts per thousand) – slightly higher than the Freshies (mean optimum across those 3 taxa = 2.9 ppt) (Wachnicka et al. 2010). Importantly, though, F. synegrotesca is generally not an indicator of a nutrient or salinity impacted site. Rather, its presence might indicate that a place is in limbo: it’s not too far gone, but it’s worse than we would expect if everything was OK. And F. synegrotesca alone doesn’t tell us much: rather, we have to look at the entire community of diatoms (and other algae and cyanobacteria) in order to make sense of the ecological impacts of modified nutrient levels and hydrology. So we use an “indicator community” analysis approach rather than “indicator species.”

As an example, in the Comprehensive Everglades Restoration Plan (CERP) Monitoring and Assessment Program (MAP) scientists from the Gaiser and Trexler labs report on how ~150 sites across the Everglades (and their animals, plants, and algae in periphyton mats) are affected by nutrient enrichment. To do this, one of the best measurements of site alteration is a combined periphyton TP-diatom community composition metric (RECOVER 2014, see pages 6-33 – 6-39). They use a “stoplight” reporting technique: green means baseline (“success”) conditions (TP < 200 µg / g), yellow means “caution” (TP = 200-250 µg / g), and red means “altered” (TP > 250 µg / g) (Fig. 3). Fragilaria synegrotesca is one of the diatoms that can contribute to a “caution” designation if it’s found away from the Freshies.

Fig. 3. Condition status of sampling sites from 2011 using a periphtyon TP-diatom community metric (from RECOVER 2014; Fig. 6-17).

So, while F. synegrotesca may seem a bit pudgy, it is a mover and shaker of the diatom scene in the Everglades. In the Everglades, there is the potential for more widespread dry-downs, human-caused phosphorus enrichment in the Everglades interior (particularly in the northern Everglades and near canals), and for sea-level rise in the southern Everglades (pushing saltier, nutrient-enriched water into the freshwater inland regions). Fragilaria synegrotesca and its associated community are great tools to understand how such disturbances are affecting the nature of this wonderful and important wetland. The ongoing diligent scientific monitoring and analysis (e.g., by CERP MAP and the Florida Coastal Everglades Long Term Ecological Research program) allow us to understand things like a potbellied diatom that inform sustainable management and conservation of the entire ecosystem.

*Ph.D. student at the Institute of Arctic and Alpine Research, University of Colorado Boulder and FIU Algae Research Lab alumnus

Gaiser, E. E., Childers, D. L., Jones, R. D., Richards, J. H., Scinto, L. J., & Trexler, J. C. (2006). Periphyton responses to eutrophication in the Florida Everglades: Cross‐system patterns of structural and compositional change. Limnology and Oceanography, 51(1part2), 617-630.

Gaiser, E. E., McCormick, P. V., Hagerthey, S. E., & Gottlieb, A. D. (2011). Landscape patterns of periphyton in the Florida Everglades. Critical Reviews in Environmental Science and Technology, 41(S1), 92-120.

Gottlieb, A., Richards, J., & Gaiser, E. (2005). Effects of desiccation duration on the community structure and nutrient retention of short and long-hydroperiod Everglades periphyton mats. Aquatic Botany, 82(2), 99-112.

Lee, S. S., Gaiser, E. E., & Trexler, J. C. (2013). Diatom-based models for inferring hydrology and periphyton abundance in a subtropical karstic wetland: Implications for ecosystem-scale bioassessment. Wetlands, 33(1), 157-173.

RECOVER (2014). System Status Report. Comprehensive Everglades Restoration Plan, Restoration Coordination and Verification (RECOVER). U.S. Army Corps of Engineers Jacksonville District, Jacksonville, Florida, and South Florida Water Management District, West Palm Beach, Florida, USA. http://141.232.10.32/pm/ssr_2014/cerp_ssr_2014.aspx

Schulte, N. (2014). Fragilaria synegrotesca. In Diatoms of the United States. Retrieved June 16, 2017, from http://westerndiatoms.colorado.edu/taxa/species/fragilaria_synegrotesca

Wachnicka, A., Gaiser, E., Collins, L., Frankovich, T., & Boyer, J. (2010). Distribution of diatoms and development of diatom-based models for inferring salinity and nutrient concentrations in Florida Bay and adjacent coastal wetlands of south Florida (USA). Estuaries and Coasts, 33(5), 1080-1098.